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
More than 10,000 relics, ____ between 5,000 to 3,000 BC, ____ at Sanxingdui since it was first discovered.
A．dating back to, was dug up
B．dating back to, have been dug up
C．dated back to, has been dug up
D．dated back to, were dug up
离心泵的性能曲线有（）条。
A、一
B、二
C、三
D、四
