油液污染的主要来源原有的污染、侵入的污染、自然新生的污染。油液因温升变质而生成的焦油、沥青等胶质渣滓及金属腐蚀后的修片属于自然新生的污染。
参考答案：对
Long,long time ago,there was a mountain.At the top of the mountain _______,in which an old monk used to tell stories.
A．a temple stood
B．is there a temple
C．stood a temple
D．a temple was there
齿轮泵按啮合方式分主要有（）两种。
A.内齿轮泵
B.外齿轮泵
C.直齿齿轮泵
D.人字齿齿轮泵
