有一铸件浇注时高度为500mm，浇注时间为20s，则浇注时型腔内金属液面的上升速度为（）。
A. 9mm/s
B. 15mm/s
C. 25mm/s
D. 30mm/s
查看答案
顶板事故按一次冒顶范围可分为（）事故两大类。
A. 冲击推垮冒顶
B. 局部冒顶
C. 推垮冒顶
D. 大面积冒顶
电子商务也是一类信息系统。
A. 正确
B. 错误
In a diesel engine, when the fuel is injected into the cylinder,（）
A. the piston is just coming upward from the bottom dead center
B. the piston has moved about one-fifth of the way up
C. the piston has moved most of the way up and almost reached the TDC
D. the piston has just past the TDC
微信支付 
支付宝支付