According to the air traffic rules youswitch off your mobile phone before bearding.A. may

问题补充：

According to the air traffic rules, youswitch off your mobile phone before bearding.

A. may

B. can

C. would

D. should

答案：

A. may

B. can

C. would

(答案→)D. should

解析：此题考查情态动词的用法。根据句意“按照空中交通规则，你应当在登机前关掉你的手机”，所以只有should(应该，应当)符合题意。