问题补充：
A good friend is a person we Turn for help when we are in trouble.
A. who
B. whom
C. to whom
D. to which
答案：
A. who
B. whom
(答案→)C. to whom
D. to which
解析：考查定语从句。句义：朋友是你处于困境时可以求救的的人。把握关键词turn，构成短语：turn to sb,向某人求救，所以用介词to ,因为先行词是a person,是人所以用whom，所以C正确。
