下拉列表中的第一个选项禁用表单上的提交按钮

我的网站上有一个连接到Mailchimp的表单。除了从下拉菜单中选择顶部选项（当前为"全部"）之外，这一切似乎都能完美工作。

这是表单的代码：

    <?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 } ?>

我也检查了下拉列表中每个选择的单独文件，但它们似乎都是正确的，所以我认为问题一定出在这段代码中。不幸的是，我无法理解它，无法解读问题可能是什么

编辑：这是网站，它是标题下面的表格。

任何帮助都非常感谢，谢谢！