html drop-down form_html/css_WEB-ITnose

I want a drop-down form like this:
---parent1---
?son1
?son2
---parent2--
?son1
?son2
---parent3--
---parent4--
---parent5--
?son1
?son2

It can be seen that it is 2 layer effect.
Cannot be linked with 2 selects. This is what customers want.

How to write html. What should this data look like?

Reply to discussion (solution)

Can optgroup meet the needs?

<select>	<optgroup label="---parent1---">    	<option>?son1</option>        <option>?son2</option>    </optgroup>    <optgroup label="---parent2---">    	<option>?son1</option>        <option>?son2</option>    </optgroup></select>

Yes. Thank you. I haven't written a dropdown form like this before. Either one or linked. I really don’t know about optgroup.