The advance and retreat of summer monsoon affects the process of rain belt in eastern Chinese mainland. The difference of summer monsoon intensity and the "disorder" of rain belt advance and retreat in China often lead to droughts and floods. If the rain belt advances slowly and lingers in the south for a long time, it will lead to "southern drought". If we advance quickly and stay in the north for a long time, it is easy to have a situation of "northern waterlogging and southern drought" Huaihe River is located in the eastern monsoon region of China, with frequent floods. Since the founding of New China, the Huaihe River Basin has been comprehensively improved and a series of water conservancy projects have been built.

So choose: a.