21. The door is low, so ______ your head when you go in.
A. attend B. guard C. protect D. mind
22. ______ in his book, he didn’t notice me when I entered the room.
A. Devoted B. Absorbed C. Absorbing D. Devoting
23.________ the sunlight for a long time is harmful to your skin.
A. Exposed to B. To expose in C. Exposing in D. Being exposed to
