单选题 It is extremely urgent that they ______ from the mountain before dark.
A. must be rescued B. should rescue C. can be rescued D. be rescued

【正确答案】 D
【答案解析】[解析] 在句型“It is urgent/necessary/important+从句”中，从句中要用虚拟语气，其构成是：should do，可以将 should省略。