And give me the correct answer only!

read the passage about butterflies.

butterflies with purple wings used to be dominant in an environment. over a period of time, it was observed that yellow butterflies were becoming the dominant color.
what is the most likely reason for the change in the butterfly population?

a. birds ate most of the sick yellow butterflies, allowing the purple to reproduce more.

b. there were fewer yellow butterflies, so they were able to reproduce at a faster rate.

c. the yellow butterflies were not eaten by birds and were able to reproduce more.

d. the purple butterflies stopped reproducing and only the yellow butterflies reproduced.