The Event That Triggers Fruit Formation Is the Growth of a Pollen

Multiple Choice

The event that triggers fruit formation is the growth of a pollen tube through the carpel of a flower. Which of the following is a reasonable hypothesis about the basis of this effect?

A) Pollen tubes grow in response to a cytokinin produced by the carpel.
B) Pollen tubes grow in response to abscisic acid.
C) The growing pollen tube produces ethylene.
D) The growing pollen tube produces auxins and/or gibberellins.