问题补充：
There are a number of cases family members or friends quarrel with each other and become enemies.
A. that
B. which
C. where
D. when
答案：
A. that
B. which
(答案→)C. where
D. when
解析：句意：又很多家庭成员和朋友彼此争吵成为敌人的情况。这里使用了定语从句，先行词是cases，定语从句中缺少的是地点状语（in the case）所以用关系副词where引导定语从句，选C。
