The dividing line between summer and autumn in the yellow calendar

Qinling Mountains and Huaihe River are important geographical boundaries in eastern China, and the 0℃ isotherm passes through here in 1 month. Where the 800 mm isoprecipitation line passes; In the temperature zone, the dividing line between warm temperate zone and subtropical zone; The dividing line between dry area and wet area, wet area and semi-wet area.

So choose: a.