问题
单项选择题
It is（）that we could only stay there for three weeks.
A.sadness
B.sadly
C.sad
D.saddened
答案
参考答案：C
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It is（）that we could only stay there for three weeks.
A.sadness
B.sadly
C.sad
D.saddened
参考答案：C
项的贡献， 则两点电荷的合电场在M和N点的强度