<script type="text/javascript">
$("#Yes").click(function() {
$("#discountselection").attr("disabled", false);
});
$("#No").click(function() {
$("#discountselection").attr("disabled", true);
});
</script>
Also, set the display style for the drop-down list or disable the property in HTML based on your standard radio selected when the page loads.
<select name="discountselection" id="discountselection" disabled="disabled">
<option value="1 Year" selected="selected">1 Year</option>
<option value="2 Years">2 Years</option>
<option value="3 Years">3 Years</option>
</select>
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