in the computer room,you can only hear keyboards___and the clicking of mice.

B 是一个被动句
and 指 keyboards being tapped 和 clicking of mice 并列
clicking of mice 也可写成 mice being clicked

在机房里，你只能听到键盘被敲打的声音和鼠标的喀嚓声。

keyboards being tapped/the clicking of mice并列，都是做hear的宾语。

因为键盘是被敲击的，所以这里用被动语态做keyboard的补足语。

B。在机房里，你只能听到键盘被敲打的声音和鼠标的喀嚓声。
hear somethingbeing done 含被动进行时 再如，
I hear her being criticized by the teacher。我听见老师在批判她

keyboards 和mice 并列，只是一个用了动名词，一个是名词结构