How to unhide a input uploader only when a user click a input selection box ?

I have on my aspx page
these
 <select name=DD1>
<option value="Nil">Pls select</option>   <option value="A">Class A</option>   <option value="B">Class B</option>   </select>
 <input type="file" name="myFile1" >     <input type="submit" value="Upload File">
These are working fine.
However, I wish the input type button to appear only when the user select the option in the upper section.
I need client side script not server side code.
Please help, thanks