从机舱通往轴隧的水密舱壁上的水密门，一般要求装设（）水密门。
A．一级
B．二级
C．三级
D．四级
参考答案：B
I _______ my homework at nine o'clock last Sunday morning. [ ]
A. am doing
B. was doing
C. do
局部安排特征包括哪几个方面？
