title: Fill multiselect dropdown depending on select
body: <p>I have a select dropdown list and a multiselect dropdown. And i want the multiselect one to be depended on the select one. How can i do it?</p>
  
  <pre><code>&lt;div class="row"&gt;
  &lt;div class="col" &gt;&lt;label&gt;Which class: &lt;/label&gt;&lt;select name="type_of_subject_c" id="type_of_subject_c" tabindex="1"&gt;
  &lt;option value="" selected="selected"&gt;--не выбрано--&lt;/option&gt;
  &lt;option value="5"&gt;5th &lt;/option&gt;
  &lt;option value="6"&gt;6th&lt;/option&gt;
  &lt;option value="7"&gt;7th&lt;/option&gt;
  &lt;option value="8"&gt;8th&lt;/option&gt;
  &lt;/select&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>And i want, for example, if a person chose 5th - show in the multiselect field such options as "Math", "English", "Literature"
  If a person chose 6th - show "Math", "Science", "Music"
  etc.</p>
  
  <pre><code>&lt;div class="row"&gt;
  &lt;div class="col"&gt;&lt;label&gt;Coruses: &lt;/label&gt;&lt;select   name="course_subj_c[]" id="course_subj_c" multiple="multiple" tabindex="1" &gt;
  &lt;option value="math"&gt;Math&lt;/option&gt;
  &lt;option value="eng&gt;English&lt;/option&gt;
  &lt;option value="lit"&gt;Literature&lt;/option&gt;
  </code></pre>