According to the air traffic rules you switch off your mobile phone before boarding.A. m

问题补充：

According to the air traffic rules, you switch off your mobile phone before boarding.

A. may

B. can

C. would

D. shall

答案：

A. may

B. can

C. would

(答案→)D. shall

解析：考查情态动词shall的用法：shall用于第二人称第三人称，可以表示警告，命令，允诺，规定，此处表示根据交通法规应尽的责任和义务。句意：根据交通法规，在登机前应该关掉手机。选D。