title: Group different selection option dropdown list
body: <p>I am not sure if this is a html or javascript tech.
  I want to achieve like that, only show two select boxes,
  once I clicked India, another select box will shows only Indiacities options  <code>name="listtwo" id="listtwo"</code>, if clicked US, shows us cities options.
  Could someone please give an example. Many thanks</p>
  
  <p>
              India
              US
              Germany
          </p>
  
  <pre><code>    &lt;select name="listtwo" id="listtwo"&gt;
          &lt;option value="Indiacity1"&gt;Indiacity1&lt;/option&gt;
          &lt;option value="Indiacity2"&gt;Indiacity1&lt;/option&gt;
          &lt;option value="Indiacity3"&gt;Indiacity1&lt;/option&gt;
      &lt;/select&gt;
  
    &lt;select name="list3" id="list3"&gt;
          &lt;option value="Germany1"&gt;Germany1&lt;/option&gt;
          &lt;option value="Germany2"&gt;Germany1&lt;/option&gt;
          &lt;option value="Germany3"&gt;Germany1&lt;/option&gt;
      &lt;/select&gt;
  
  &lt;select name="list4" id="list4"&gt;
          &lt;option value="US1"&gt;US1&lt;/option&gt;
          &lt;option value="US2"&gt;US1&lt;/option&gt;
          &lt;option value="US3"&gt;US1&lt;/option&gt;
      &lt;/select&gt;​
  </code></pre>