选择标签下的选择标签

Question

我该怎么做？This is a sample image of the output I want在此先感谢。刚开始学习新事物

<form>
<select>
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3
        <select>
            <option value="3.1">3.1</option>
            <option value="3.2">3.2</option>
        </select>
    </option>
</select></form>