In the spring of 20 14, smog weather continued in many places in northern China, and the pollution index of some cities exceeded the upper limit of measurement. The PM2.5 monitoring point is "broken",

In the spring of 20 14, smog weather continued in many places in northern China, and the pollution index of some cities exceeded the upper limit of measurement. The PM2.5 monitoring point is "broken", and the air is comparable to poison. A, setting off fireworks will produce a lot of harmful gases such as CO, SO2 and dust, which will pollute the air and increase the PM2.5 in the atmosphere, which is wrong;

B, automobile exhaust contains smoke and dust produced by insufficient fuel combustion, which will produce particles with a diameter less than or equal to 2.5 microns, so it can cause an increase in PM2.5 in the atmosphere, which is wrong.

C, units with large air pollutant emissions consciously reduce emissions, which can reduce PM2.5 in the atmosphere, so it is correct;

D, a large number of coal as fuel will produce harmful gases such as CO, SO2 and dust, which will pollute the air and increase PM2.5 in the atmosphere. This is wrong.

So choose C.