按推力轴承的支撑结构划分，当前推力轴承的型式主要有（）种。
A.1；
B.2；
C.3；
D.4；
参考答案：C
The film used in photography is only a fraction of an inch thick, but is composed of many layers.
A．one tenth
B．layer
C．bit
D．group
血小板的作用是（）.
A、止血、凝血
B、造血
C、防御
