滤线栅表面平均1cm2中铅的体积（cm3）称（）
A.栅比
B.栅焦距
C.栅密度
D.铅容积
E.聚焦栅距离界限
参考答案：D
He often does some washing _____ Sunday. [ ]
A. at
B. in
C. on
D. by
油井出水有什么危害？
