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<select id="myDropdown" onchange="updateContent()">

 <option value="">Select one</option>
 <option value="one">Option 1</option>
 <option value="two">Option 2</option>
 <option value="three">Option 3</option>

</select>

 Pick something above

<script> function updateContent() {

 var value = document.getElementById("myDropdown").value;
 var output = document.getElementById("output");

 if (value === "one") {
   output.innerHTML = "This is content for Option 1";
 } else if (value === "two") {
   output.innerHTML = "This is content for Option 2";
 } else if (value === "three") {
   output.innerHTML = "This is content for Option 3";
 } else {
   output.innerHTML = "Pick something above";
 }

} </script>