At the top of the high mountains, there is very _____ oxygen in the air.
-How about having a picnic with me?-I’d love to, ______ I’m afraid I have no time. A. so B. and C. but
-Dad, may I watch TV?-No, you can’t. You _______ finish your homework first.A. must B. can C. may
-Oh, so many people in the park?-Nobody likes to stay at home ______ Sunday morning. A. in B. on C. at
-How much is the pair of shoes?-Twenty dollars ____ enough.A. is B. are C. am
-Do you mind my opening the window?-______. Please do it now. A. Of course not. B. Yes, I do. C. Certainly