在开式传动中齿轮主要的失效形式是（）。
A.齿面胶合
B.齿面点蚀
C.轮齿塑性变形
D.齿面磨损
参考答案：D
渗水井顶部四周用黏土筑堤围护，井顶应加筑混凝土盖，严防渗井淤塞，渗井离路堤坡脚不应小于 ( )m。
A．8．
B．10．
C．18．
D．20
Admiral Cervera knew he was being ordered to certain destruction but felt compelled to obey. He chose the morning of July 3 for a gallant escape attempt.
A. daring B. futile C. critical D. fruitful
