21．As the immune system weakens, it loses the ability ．illnesses
A．fighting B．fought C．to fight D．to be fought
22．—Could you lend me your watch for my exam tomorrow?
—I’m sorry, but mine ．in the shop
A．is being repaired B．is repairing
C．was repaired D．was repairing
23．Alexander tried to get his work ．in the medical circles
A．to recognize B．recognizing
C．recognize D．recognized
