How to implement a secondary vertical drop-down menu in bootstrap3.3.4? _html/css_WEB-ITnose

I want to implement a two-level vertical drop-down menu. How to implement it: click on user management, 4 sub-menus will pop up, click again and they will shrink back. File management is also similar

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The drop-down list is placed in a list. The display is none at the beginning, so it does not take up space. When user management is clicked, the release function changes the display Just change it to block

The drop-down list is placed in a list, and the display is none at first, so it doesn't take up space. When you click on the user management, just change the display to block

I used the source code on the Internet to get one, but the text in the secondary menu cannot be centered