How to Fairly Compare Disease Frequencies Between Groups
1. a and d only
A common cause of death (ex. CVD) is less common in the factory workers (they may tend to get more exercise) than in the general population.
Disease X becomes more virulent (kills more frequently) but less common.
Some other common cause of death declines in the factory workers.
Death from disease X and all other causes becomes more frequent in factory workers without changing in the general population. (Because CFR did not change there must be an increase in the number of cases of disease X without change in the proportion that die.)