帮忙分析个英语单选题
题目
帮忙分析个英语单选题
By then,the flood that had happened in Bishan country of Chongqing ___ to the inner city,making it become an isolated
island.
A has B had been spread C has spread D has been spread
并且分析下时态,
答案
B, by then 到×时候为止,后面有had happened, 过去完成,保持时态一致,过去完成时,had^been spread, been这里不是被动哦
举一反三
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．
我想写一篇关于奥巴马的演讲的文章,写哪一篇好呢?为什么好
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