A Study Was Done to Compare the Lung Capacity of Coal

Multiple Choice

A study was done to compare the lung capacity of coal miners to the lung capacity of farm workers. The researcher studied 200 workers of each type. Other factors that might affect lung capacity are smoking habits and exercise habits. The smoking habits of the two worker types are similar, but the coal miners generally exercise less than the farm workers
-Which of the following is a confounding variable in this study?

A) Exercise
B) Lung capacity
C) Smoking or not
D) Occupation