Drop Down menu Validation with Alert

If drop down menu is left unanswered provide an alert to the user.
I already have the code for providing the alert for an unanswered drop down menu. However, Im trying to add this code to my existing Javascript.

The code creates a form that gathers information regarding users. The form has 5 steps:

  1. User information (Name, email, phone, age, gender)
  2. Yes or No question (Drop down menu)
  3. Yes or No question (Drop down menu)
  4. Yes or No question (Drop down menu)
  5. Yes or No question (Drop down menu)

The code also has embedded animation for every time the user presses the next/previous button.

How do I add my drop down menu validation alert, to my existing Javascript (for steps 2-5). Please visit http://thecodeplayer.com/walkthrough/jquery-multi-step-form-with-progress-bar
for better understanding of what the animation looks like.

var current_fs, next_fs, previous_fs; //fieldsets
var left, opacity, scale; //fieldset properties which we will animate
var animating; //flag to prevent quick multi-click glitches

$(".next").click(function(){

//text inputs
if(!document.getElementById('fullname').value){
alert('Full Name is required');
return false;
}

else if(!document.getElementById('email').value){
alert('Email is required');
return false;
}

else if(!document.getElementById('phone').value){
alert('Phone Number is required');
return false;
}

else if(!document.getElementById('age').value){
alert('Age is required');
return false;
}
        
//radio buttons
var genderSet = false;
var genderBtns = document.getElementsByName('gender');
//console.log(genderBtns);
for(var i=0, btn; btn=genderBtns[i];++i){
if(btn.checked){
  genderSet=true;
  break;
 }
 }
 if(!genderSet){
 alert('Gender is required');
 return false
 } 
      
	if(animating) return false;
	animating = true;
	
	current_fs = $(this).parent();
	next_fs = $(this).parent().next();
    	
	//activate next step on progressbar using the index of next_fs
	$("#progressbar li").eq($("fieldset").index(next_fs)).addClass("active");
	
	//show the next fieldset
	next_fs.show(); 
	//hide the current fieldset with style
	current_fs.animate({opacity: 0}, {
		step: function(now, mx) {
			//as the opacity of current_fs reduces to 0 - stored in "now"
			//1. scale current_fs down to 80%
			scale = 1 - (1 - now) * 0.2;
			//2. bring next_fs from the right(50%)
			left = (now * 50)+"%";
			//3. increase opacity of next_fs to 1 as it moves in
			opacity = 1 - now;
			current_fs.css({'transform': 'scale('+scale+')'});
			next_fs.css({'left': left, 'opacity': opacity});
		}, 
		duration: 800, 
		complete: function(){
			current_fs.hide();
			animating = false;
		}, 
		//this comes from the custom easing plugin
		easing: 'easeInOutBack'
	});
});

$(".previous").click(function(){
	if(animating) return false;
	animating = true;
	
	current_fs = $(this).parent();
	previous_fs = $(this).parent().prev();
	
	//de-activate current step on progressbar
	$("#progressbar li").eq($("fieldset").index(current_fs)).removeClass("active");
	
	//show the previous fieldset
	previous_fs.show(); 
	//hide the current fieldset with style
	current_fs.animate({opacity: 0}, {
		step: function(now, mx) {
			//as the opacity of current_fs reduces to 0 - stored in "now"
			//1. scale previous_fs from 80% to 100%
			scale = 0.8 + (1 - now) * 0.2;
			//2. take current_fs to the right(50%) - from 0%
			left = ((1-now) * 50)+"%";
			//3. increase opacity of previous_fs to 1 as it moves in
			opacity = 1 - now;
			current_fs.css({'left': left});
			previous_fs.css({'transform': 'scale('+scale+')', 'opacity': opacity});
		}, 
		duration: 800, 
		complete: function(){
			current_fs.hide();
			animating = false;
		}, 
		//this comes from the custom easing plugin
		easing: 'easeInOutBack'
	});
});

$(".submit").click(function(){
	return false;
})
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<!-- fieldsets -->
	<fieldset>
		<h2 class="fs-title">Step 1</h2>
		<h3 class="fs-subtitle">Background Information</h3>
        <input type="text" id="fullname" name="fullname" placeholder="Full Name">
        <input type="text" id="email" name="email" placeholder="E-Mail">
        <input type="text" id="phone" name="phone" placeholder="Phone">
        <input type="number" id="age" name="age" placeholder="Age">
        
        <h4>Gender</h4>
        <div class="row">
        <div>
          <input type="radio" name="gender" value="male" id="gender-male"/>
          <label for="gender-male">Male</label>
        </div> 
          <div>
          <input type="radio" name="gender" value="female" id="gender-female"/>
          <label for="gender-female">Female</label>
        </div>
            
    </div>
        <div class="row">
      <h4>Description</h4>
      <div class="input-group">
        <label for="terms">Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Consectetur adipiscing elit, sed do eiusmod tempor. </label>
      </div>
    </div>

<input type="button" name="next" id="btnNext" class="next action-button" value="Next" />
	</fieldset>
    
    
    
	<fieldset>
		<h2 class="fs-title">Step 2 </h2>
		<h3 class="fs-subtitle">Please select one of the following</h3>
        
        Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.  <br>
        <div>
<select name="past" id="past">
  <option value=""disabled selected>Select One</option>
  <option value="a">Yes</option>
  <option value="b">No</option>
</select>
         </div>
        <br> 
		<input type="button" name="previous" class="previous action-button" value="Previous" />
		<input type="button" name="next" class="next action-button" id="PastNext" value="Next" />
	</fieldset>
    
    
<fieldset>
		<h2 class="fs-title">Step 3</h2>
		<h3 class="fs-subtitle">Please select one of the following</h3>
    
   Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 
		   <div>
<select>
  <option value=""disabled selected>Select One</option>
  <option value="a">Yes</option>
  <option value="b">No</option>
</select>
         </div>
        <br> 
		<input type="button" name="previous" class="previous action-button" value="Previous" />
		<input type="button" name="next" class="next action-button" value="Next" />
	</fieldset>
<fieldset>
		<h2 class="fs-title">Step 4</h2>
		<h3 class="fs-subtitle">Please select one of the following</h3>
    
    Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 
    <br> 
    
<div>
<select>
  <option value=""disabled selected>Select One</option>
  <option value="a">Yes</option>
  <option value="b">No</option>
</select>
         </div>
        <br> 
		<input type="button" name="previous" class="previous action-button" value="Previous" />
		<input type="button" name="next" class="next action-button" value="Next" />
	</fieldset>
	<fieldset>
		<h2 class="fs-title">Step 5 </h2>
		<h3 class="fs-subtitle">Please select one of the following</h3>
    
    Can you come to this location?<br> <br>
    <br> 
        
        
<div>
<select>
  <option value=""disabled selected>Select One</option>
  <option value="a">Yes</option>
  <option value="b">No</option>
</select>
         </div>
        <br><br><br>
        
        <div class="row">
      <h4>Terms and Conditions</h4>
      <div class="input-group">
          <input id="terms" type="checkbox">
        <label for="terms">Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. </label>
      </div>
    </div>
        
		<input type="button" name="previous" class="previous action-button" value="Previous" />
		 <input type="submit" name="submit" class="submit action-button" value="Submit" />
	</fieldset>

DROP DOWN MENU ALERT CODE:

$("#PastNext").click(function(event){
         var validate = $("#past").val();
         if(validate=="")
         {
             event.preventDefault();
             alert("You have not selected any option");
         } 

 });

Right before if(animating) return false;
like this:

//First select
if($("#past").val()=="")
{
   alert("You have not selected any option");
   return false;
} 
//Next select
else if($("#past2orwhateverthenameis").val()=="")
{
   alert("You have not selected any 2nd option");
   return false;
}

Do for all 5.