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

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. should

答案：

A. may

B. can

C. would

(答案→)D. should

解析：should此处表示根据交通法规应尽的责任和义务。