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

镜下脓尿是指尿液中的白细胞超过

A．1个／HPF 

B．2个／HPF 

C．3个／HPF 

D．5个／HPF 

E．7个／HPF