We must keep our room clean. Dirt and disease go , you know.

A. on B. with

C. at D. in

D本题涉及虚拟语气的使用，从if引导的从句分析可以知道与过去相反，should have done表示本来不应该但实际上干了，不符合题意。

A. as B. while

C. the D. the more

A. used to B. am used to it

C. sued D. used to do

A. them to watch

B. that they watch

C. their watching

D. watching

A. Gate 21 B. 21St Gate

C. the Gate 21 D. 21 Gate

A. nearly B. mostly

C. almost D. possibly

A. While B. Once

C. Though D. Unless

A. his B. your

C. their D. one's

A. You speak

B. That's it

C. It’s up to you

D. You got it