______ hearing his father is dead,he feels very sad. [ ]
A. At
B. In
C. Upon
D. To
答案:C
将钢件加热到临界点以上30~50℃保温一定时间,然后在()中冷却的热处理过程称为正火。
调整脱轨自动制动阀的拉环、顶梁与车轴的间隙值需在()上进行。
A.曲线
B.坡道
C.平道
D.弯道