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<html><body><select id="pets" size="3"> <option>Cat</option> <option>Dog</option> <option>Horse</option> </select><p>Click the button to disable the third option (index 2) in the dropdown list.</p><button onclick="myFunction()">Disable Option</button><script>function myFunction() { var x = document.getElementById("pets").options[2].disabled = true;}</script></body></html>