When the girl ___ a teacher came into the classroom
题目
When the girl ___ a teacher came into the classroom
A sang
B sings
C to sing
D was singing
答案
When the girl _D was singing__ a teacher came into the classroom
当女孩在唱歌时,老师走进了教室.
主句动作发生时,从句动作sing在进行,所以用过去进行时
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已知函数f（x）=x，g（x）=alnx，a∈R．若曲线y=f（x）与曲线y=g（x）相交，且在交点处有相同的切线，求a的值和该切线方程．
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