汽车差速器中的行星轮在转弯时( )
A. 有自转
B. 无自转
C. 固定不动
D. 仅有公转
为消除零件因偏重而引起的振动，必须进行()。
A. 平衡试验
B. 水压试验
C. 气压试验
D. 密封试验
For what reason does Frank ask for sick leave?[音频]
A. He has caught a cold.
B. He didn't sleep well last night.
C. He has got a headache.
Which of the following is true about Frank’s mother?
A. She works in a hospital.
B. She is in hospital because of a heart attack.
C. She has spent a week with Frank since she was ill.
微信支付 
支付宝支付