It worries the couple a lot their only son is behaving worse and worse these days.A. howB

问题补充：

It worries the couple a lot their only son is behaving worse and worse these days.

A. how

B. why

C. that

D. what

答案：

A. how

B. why

(答案→)C. that

D. what

解析：考查主语从句，这里it是形式主语，that是引导主语从句，主语从句中不缺成分，用that引导，句意：他们唯一的儿子最近表现越来越差让这对夫妇很担心。选C。