18. Doing your homework is a sure way to improve your test scores, and this is especially true ________ it comes to classroom tests.
A. as B. since C. when D. after
19. I have been keeping that photo ________ I can see it every day, as it always reminds me of my college days.
A. which B. where C. whether D. when
20. You may borrow this book ________ you promise to give it back.
A. in case B. so long as C. as if D. even if
From: http://www.gaozhongzuowen.cn/yingyu/