解答英语问题！！
_____________the unusually bad weather last summer, there would not have been such worse floods along the Yangtze River.
A. It had not been for
B. Had it not been for
C. It had not been
D. Had it not been
如何选择？为什么？
参考答案:B. Had it not been for if 引导的非真实条件句 ,和过去事实相反,省略if , 句子要倒装