Among the following surveys, (a) Understand the annual per capita income of residents in Yinzhou District and (b) Understand the weather forecast of Yinzhou TV Station.

A, understand the annual per capita income of residents in Yinzhou District, with a large number of people and heavy workload, which is suitable for sampling survey, so the choice is wrong;

B, given the weather forecast ratings of Yinzhou TV station, the census is of little significance and suitable for sampling survey, so the selection is wrong;

C, I learned that a middle school in Yinzhou District failed the math period, and the workload was small and lossless, so it was suitable for the general survey, so the choice was correct;

D, understand the population flow leaving Yinzhou District on a certain day, the number is large and the scope is wide, so it is appropriate to take spot checks, so the choice is wrong.

So choose: C.