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function showNestedDropdown() {
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          var nestedDropdownContainer = document.getElementById("nestedDropdownContainer");
      
          if (firstDropdown.value !== "") {
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          } else {
            nestedDropdownContainer.style.display = "none";
          }
        }

    <select id="firstDropdown" onchange="showNestedDropdown()">
        <option value="">Select an option</option>
        <option value="option1">Option 1</option>
        <option value="option2">Option 2</option>
      </select>
      
      <div id="nestedDropdownContainer" style="display: none;">
        <select id="nestedDropdown">
          <option value="">Select a nested option</option>
          <option value="nested1">Nested Option 1</option>
          <option value="nested2">Nested Option 2</option>
        </select>
      </div>