In a perfectly competitive constant-cost industry:

A. factor prices fall as industry output increases.
B. factor prices do not change as industry output increases.
C. there is no way to predict what will happen to factor prices as industry output increases.
D. factor prices rise as industry output increases.

Because people did not know that yellow fever was caused by the bite of an infected mosquito, they assumed incorrectly that people could catch the disease through touch. To help control the disease, letters were cleaned with vinegar before being sent. This did not work, because it was a false cause-and-effect relationship.