Classic ASP - Function to check that the user has selected a value

I am trying to update one of my asp pages so that it forces the user to select a value in a drop down list box. Currently it defaults to one of the values but users then never change it, I would like to force them to choose one.

What is the easiest simplest way to do this?

<td align="right" nowrap>when would you like us to call you?&nbsp;&nbsp;</td>
                  <td align="left"> <select class="inputfield" name="TimeRequired">
                      <!--<option value="0" <%if Cstr(callerRequestTime) = "0" then response.write("selected")%>>Immediately!</option>-->
			<option value="15" <%if Cstr(callerRequestTime) = "15" then response.write("selected")%>>in 
                      15 minutes</option>
                      <option value="30" <%if Cstr(callerRequestTime) = "30" then response.write("selected")%>>in 
                      30 minutes</option>
                      <option value="60" <%if Cstr(callerRequestTime) = "60" then response.write("selected")%>>in 
                      one hour</option>
                    </select> </td>

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