问题补充：
Today, we’ll discuss a number of cases parents have much difficulty in communicating with their children.
A. which
B. that
C. where
D. why
答案：
A. which
B. that
(答案→)C. where
D. why
解析：句意：今天，我们要讨论一些案例，在这些案例中，父母与孩子交流有困难。where引导定语从句，修饰cases，在从句中做地点状语，表示在这些案例中。
