单选题 Mother insisted that ______.

A、 they are to be back before nine in the evening
B、 they ought to be back before nine in the evening
C、 they be back before nine in the evening
D、 they had to be back before nine in the evening

【正确答案】 C

【答案解析】insist表示“坚决要求”。其后的宾语从句中要用虚拟语气，其构成是should do。