氢冷发电机的密封油作用是（）
A.密封
B.润滑，冷却
C.密封，润滑，冷却
D.密封，润滑
参考答案：C
What's in the classroom?[ ]
A. It's a picture.
B. It's black.
C. A board, two lights, many desks and chairs.
You _________ never play in the street. It's not safe.
A．can
B．may
C．must
D．need
