问题
选择题
He ______ his friend the moment he got the good news.
A contacted with B contacted C kept in contact with D noticed
答案
答案：B
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He ______ his friend the moment he got the good news.
A contacted with B contacted C kept in contact with D noticed
答案：B
，方向如图，如果切断其中一根导线使正方形ABCD的中心O点的磁感应强度最大，则应切断
