What Was One Study That Led to the Conclusion That

What was one study that led to the conclusion that sperm cells were necessary for fertilization?

A) Leeuwenhoek saw fertilization of an egg by a sperm.
B) Male frogs wearing little trousers were able to mate but did not produce any offspring
C) Male fruit flies that were castrated could mate but could not produce offspring.
D) Female fruit flies were fooled into mating with other female fruit flies, and no offspring resulted.
E) Sterilized frog semen, when mixed with frog eggs, did not result in fertilization.