title: jquery, get id of dropdownlist when button is clicked
body: <p>My dom looks like:</p>
  
  <pre><code>&lt;td&gt;
  
  &lt;select id=s1 class=c1&gt;...&lt;/select&gt;
  &lt;select id=s2 class=c1&gt;...&lt;/select&gt;
  &lt;select id=s3 class=c1&gt;...&lt;/select&gt;
  &lt;input type=button id=btn value=click/&gt;
  &lt;/td&gt;
  </code></pre>
  
  <p>Now when the page loads, I do a <code>$(".c1").hide();</code> and then based on other logic, make one of them visible.</p>
  
  <p>Now when the button is clicked, I want the ID of the drop down list that is currently visible.</p>
  
  <p>How can I do this?</p>