问题
选择题
It is written clearly in the requirements that all passengers _______ check onto board thirty minutes before the plane takes off．
A．may
B．can
C．will
D．shall
答案
答案：D
四个选项分别表示可以、能、将要和应该，根据题意选D，要求中清晰地写明了，飞机起飞前三十分钟，所有乘客都应该检查后登机。