大、中型柴油机的活塞的冷却多采用（）方式，以提高对活塞顶的冷却效果。
A．自由喷射冷却
B．径向散热冷却
C．循环冷却
D．振荡冷却
参考答案：D
When I called you this morning, nobody answered the phone, ?
A．where were you
B．where did you go
C．where have you been
D．where have you gone
为什么老年人忌用力排便？
