卸储煤设备多采用开式齿轮传动,齿轮失效的主要原因是()。
A.齿面磨损
B.应力集中
C.疲劳点蚀
D.胶合
参考答案:A
He was ______ about his ______ son, who is always skipping classes.
A.worried, worried
B.worrying, worried
C.worried, worrying
D.worrying, worrying
蛇:蛙:鱼()
A.春:夏:秋
B.大米:玉米:高粱
C.传感:通信:计算机
D.煤:石油:天然气