To investigate the following questions, what do you think is the inapplicable sampling survey?

A, testing the air quality in Tianjin is suitable for spot check, so this option is wrong;

B, understanding the physical exercise situation of middle school students in our city is suitable for spot check, so the option is wrong;

C, Binhai New Area recruitment, interview candidates suitable for census, so this option is correct;

D. The survey on residents' support for smoking ban in our city is suitable for sampling survey, so the option is wrong;

So choose: C.