First option in dropdown disables submit button on form

First option in dropdown disables submit button on form

I have a form on my site that is hooked up to Mailchimp. It all seems to
work perfectly apart from if the top option (currently 'all') is selected
from the dropdown menu.
Here is the code for the form:
<?php if(is_front_page()) { ?>
<style>
.fixed_header.social_header #main{padding-top: 0px!important;}
.mailchimp{ width: 100%;background: url(<?php bloginfo('template_url')
?>/images/bg_mc.png) no-repeat; background-position: left top;
background-size: 100% 100% ; min-height: 399px; padding: 154px 0 20px 0;}
.inner-mailchimp{margin: 0 auto; width: 805px; text-align: center;}
.mailchimp h1, .mailchimp h3{color: #fff;}
/*#news_mc, #everything, #business_mc, #technology_mc, #dnd_mc,
#smedia_mc{display: none;}*/
#mc_embed_signup{background: none!important;}
#everything_mc{ display: block;}
.form_mc{width: 400px; float: left;}
#mc_embed_signup .mc-field-group select{ padding: 7px 0!important;}
#dnd_mc, #news_mc, #business_mc, #technology_mc, #smedia_mc{display:none;}
.mc-field-group label{color: #fff;}
#mc-embedded-subscribe-form .clear{visibility: visible; overflow: visible;
right: -271px; top: -13px; background: url(<?php
bloginfo('template_url');?>/images/c.png ); position: relative;}
#mc_embed_signup .button{ background-color: #005CBF!important;
font-weight: normal!important;}
</style>
<script>
function fill_email(my_mail){
var fillupemail = document.getElementById(my_mail).value;
//alert(fillupemail);
document.getElementById('unique_news').value = fillupemail;
document.getElementById('unique_business').value = fillupemail;
document.getElementById('unique_dnd').value = fillupemail;
document.getElementById('unique_everything').value = fillupemail;
document.getElementById('unique_smedia').value = fillupemail;
document.getElementById('unique_technology').value = fillupemail;
}
function mailchange(id){
var x = document.getElementById(id);
if(x.value == 'everything'){
document.getElementById('dnd_mc').style.display='none';
document.getElementById('news_mc').style.display='none';
document.getElementById('business_mc').style.display='none';
document.getElementById('technology_mc').style.display='none';
document.getElementById('smedia_mc').style.display='none';
document.getElementById(x.value+'_mc').style.display='block';
}
if(x.value == 'news'){
document.getElementById('everything_mc').style.display='none';
document.getElementById('dnd_mc').style.display='none';
document.getElementById('business_mc').style.display='none';
document.getElementById('technology_mc').style.display='none';
document.getElementById('smedia_mc').style.display='none';
document.getElementById(x.value+'_mc').style.display='block';
}
if(x.value == 'business'){
document.getElementById('everything_mc').style.display='none';
document.getElementById('dnd_mc').style.display='none';
document.getElementById('news_mc').style.display='none';
document.getElementById('technology_mc').style.display='none';
document.getElementById('smedia_mc').style.display='none';
document.getElementById(x.value+'_mc').style.display='block';
}
if(x.value == 'technology'){
document.getElementById('everything_mc').style.display='none';
document.getElementById('dnd_mc').style.display='none';
document.getElementById('news_mc').style.display='none';
document.getElementById('business_mc').style.display='none';
document.getElementById('smedia_mc').style.display='none';
document.getElementById(x.value+'_mc').style.display='block';
}
if(x.value == 'dnd'){
document.getElementById('everything_mc').style.display='none';
document.getElementById('news_mc').style.display='none';
document.getElementById('business_mc').style.display='none';
document.getElementById('technology_mc').style.display='none';
document.getElementById('smedia_mc').style.display='none';
document.getElementById(x.value+'_mc').style.display='block';
}
if(x.value == 'smedia'){
document.getElementById('everything_mc').style.display='none';
document.getElementById('dnd_mc').style.display='none';
document.getElementById('news_mc').style.display='none';
document.getElementById('business_mc').style.display='none';
document.getElementById('technology_mc').style.display='none';
document.getElementById(x.value+'_mc').style.display='block';
}
}
</script>
<div class="mailchimp">
<div class="inner-mailchimp">
<h1>Scientia potentia est</h1>
<h3>Knowledge is Power</h3>
<div class="form_mc">
<?php include('mailchimp-everything.php'); ?>
<?php include('mailchimp-news.php'); ?>
<?php include('mailchimp-business.php'); ?>
<?php include('mailchimp-technology.php'); ?>
<?php include('mailchimp-dnd.php'); ?>
<?php include('mailchimp-smedia.php'); ?>
</div><!-- form -->
<div class="form_mc">
<div class="everything_mc display_mc" id="mc_embed_signup"><form>
<div class="mc-field-group selectmail_mc">
<label for="mce-EMAIL">Interest </label>
<select onchange="mailchange(this.id)" id="selectmail_c">
<option value="everything">All</option>
<option value="news">News</option>
<option value="business">Business</option>
<option value="technology">Technology</option>
<option value="dnd">Design & Development</option>
<option value="smedia">Social Media</option>
</select>
</div></form>
</div>
</div><!-- form -->
<small style="color: #FFFFFF; float: left; font-size: 13px;
margin-bottom: 20px; text-align: center; width: 100%;">Don't worry
there's no spam just intriguing posts and <u>free</u>
resources.</small>
</div>
</div>
<?php } ?>
I have checked through the separate files for each selection on the
drop-down too but they all seem correct so I believe the problem must be
in this code. Unfortunely I can't understand it enough to decipher what
the problem could be.
Any help is much appreciates, thanks!