Which of the following characterizes smog? 

A)  Industrial smog consists of a mixture of sulphur dioxide, suspended droplets of sulphuric acid, and a variety of suspended solid particles. 
B)  All industrial countries now use modern technologies to prevent industrial smog. 
C)  In the United States, grey-air smog was less of a problem 30 years ago than it is now. 
D)  Industrial smog is a problem primarily in the spring.