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【单选题】
沿y轴放置的电介质圆柱底面积为S,下底面坐标为a,上底面坐标为b,周围是真空,已知电介质内各点极化强度 (其中K 为常量,j为沿y轴正向的单位矢量),则圆柱上底面上的极化电荷面密度()。A.
Kb
B.
Ka
C.
-Kb
D.
-Ka
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【简答题】The secret of s successful school __________ effective co-operation between professional leadship, the goverment and the local authority.A. lie down B. lie on C. lie to D. lie in
【单选题】The secret of a successful school consists in effective co-operation between professional leadership, the government and the local authority.
A.
lies down
B.
lies with
C.
lies in
D.
lies ahead
【简答题】制备聚乙烯醇的单体是什么?为什么?请写出制备聚乙烯醇工艺流程及各化学反应方程式。
【单选题】The successful operation of any engine or machine and ____ depend on effective lubrication.
A.
their no trouble period
B.
their freely trouble period
C.
its free trouble life
D.
its trouble-free life
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