航空螺钉是按哪种精度等级制造的：（）.
A、2-自由配合
B、3-中度配合
C、4-紧配合
D、1－松配合
参考答案：A, B
Jack took music than any boy in his class and held a concert successfully.
A．serious
B．seriously
C．more serious
D．more seriously
变压器油的作用是（）。
A.润滑
B.导电
C.绝缘和冷却
