What Data Allowed Bastock to Conclude That a Single Gene

What data allowed Bastock to conclude that a single gene mutation affected fruit fly behavior?

A) Mutant flies had fewer pheromone receptors than wild-type flies
B) Mutant flies obtained fewer matings than wild-type flies
C) Mutant flies were a different color than wild-type flies
D) Mutant flies displayed less courtship behavior than wild-type flies
E) Mutant flies were less common in nature than wild-type flies