Dropdown Navigation

I want to develop for navigation drop-down with css but i can’t find code

example:

<div id="menu">
	<ul>
	<li><a href="#link">Link 1</a>
	<ul>
	<li><?php wp_dropdown_categories( $args ); ?></li>
	</ul>
	</li>
     </ul>
     </div>

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