21. Senior 3 high school students should be given enough sleeping time,_______ they are busy preparing for exams to enter college.
A. even so B. as though
C. as if D. even if
【解析】选D。 even if“即使”。 句意：高三的学生应该给予足够的睡眠，即使他们忙着准备高考。 even so不是连词，不引导从句，as though/if好像，似乎，与句意不符。
