(20 13? Since 20 13 and 1 June, the central and eastern regions have been caught in severe smog pollution, as shown in the figure. The Central Meteorological Observatory has turned the fog into blue.

A, increasing the intensity of changing coal into electricity and gas, and promoting the use of clean energy can improve air quality;

B, strengthening the renewal of old motor vehicles and improving exhaust emission standards can improve air quality;

C, reducing students' outdoor exercise time and avoiding respiratory diseases has nothing to do with improving air quality;

D, limiting the setting off of firecrackers in festivals and reducing the emission of harmful gases can improve air quality.

So choose C.