I see some children _____ on the street.[ ]
A. walked
B. walks
C. walking
答案：C
— Is Jack on duty today?
— It _____ be him. It's his turn tomorrow. [ ]
A. mustn't
B. won't
C. can't
D. needn't
污水排水系统设置的界限称为( )。
A．排水区界 B．排水界限 C．排水定额 D．排水极限
