活塞的对中及活塞杆的径向跳动有何影响？
参考答案：
活塞的对中不好或活塞杆的径向跳动值过大会造成活塞环和填料环的偏磨，使泄漏量增大，缩短使用寿命。
﹣3﹣（﹣5）=（ ）．
Mr Green, together with his son,_____supper at 6:00 yesterday evening.
[ ]
A. are eatting
B. is eatting
C. were eating
D. was eating
