rainy season

Frontal rain refers to the rain produced by the meeting of cold air mass and warm air mass, and the warm air mass is lifted. The eastern part of China is a typical monsoon climate zone. In May and June every year, when the cold air mass and the warm air mass meet in the middle and lower reaches of the Yangtze River, rainy weather will occur, which is called the rainy season. Therefore, option a meets the meaning of the question; Options b, c and d don't meet the meaning of the question.

So choose: a.