Don’t forget _______ the door when you ________.

A. closing；leave B. closing；are leaving C. to close；leave D. to close；will leave

C

句意：当你离开时，不要忘了关门。动词forget to do,意为“忘记做某事”,表示该事还没做；forget doing忘记做过的事情；而when引导的时间状语从句用一般现在时表示将来。