1．They ________ have arrived at lunchtime but their flight was delayed.
A. will B. can
C. must D. should
[解析]句意：他们本应该午餐时间到达，可是他们的飞机晚点了。can 用在疑问句和否定句中，意为：可能；must have done 表示推测，意为：一定做过；should have done 表示(过去)本应该做而没做。根据句意可知答案为D项。
[答案]D
2．If you ________ smoke, please go outside.
A. can B. should
C. must D. may
[解析]句意：如果你非要抽烟，请到外面去。must 用于条件句，表示“非要，偏要”。其他三项均不符合语境。
[答案]C
