As winter approaches, temperatures begin to drop. The days grow shorter, and squirrels are seen gathering nuts and burying them. Birds fly in flocks toward the south, but some animals, like mountain hares, do not mind the cold and instead adapt by turning their fur white. Some insects, like the morning cloak butterfly, seek shelter in tree holes or other shelters. Then, when temperatures slowly climb back up, many animals return to their homes and come out of hiding. Which of the following is the main external stimulus in the passage?

A.
The changing seasons
B.
The animals leaving
C.
The animals hiding
D.
The diversity of animals