The weather system that causes the "Meiyu" weather phenomenon in the middle and lower reaches of the Yangtze River is () A. Cyclone B. Typhoon C. Front D. Anti-gas.

A, typhoons along the southeast coast of China are mainly formed by tropical cyclones in summer and autumn, which does not conform to the meaning of the question;

B, the typhoon is formed by cyclone, which mainly occurs in the southeast coastal areas of China, which does not meet the meaning of the question;

C, "Meiyu" in the middle and lower reaches of the Yangtze River is formed by a quasi-static front, so it is correct;

D. Anti-cyclone (high pressure) systems with sinking central airflow usually bring sunny weather. The summer drought in the middle and lower reaches of the Yangtze River in mid-July was caused by an anticyclone, which did not meet the question.

So choose: C.