HTML。隐藏/显示下拉菜单，具体取决于是否在另一个下拉菜单中选择了一个选项

I am trying to have originally just one drop down menu when a website loads. Lets say that drop down menu only has two options "A" and "B". If the user selects option "A" I want another drop menu then to appear on the website (just below the original). If the user selects option "B" I want a different menu to appear below the original. I am also using PHP to make things even more complicated. Can anyone guide me on how I can accomplish this?I am trying to have originally just one drop do