All living things must be able to adapt to their environments in some way in order to survive. As external environments change, like with seasons, the internal conditions of living organisms must change as well. These internal changes allow organisms to maintain temperature, hydration, and overall health. Which of these is the best example of a living thing showing this characteristic?

A) A mother lion nurses her cubs in the safety of a den.
B) A bear's heart rate lowers to conserve energy during winter.
C) A pupa forms around a caterpillar as it morphs into a butterfly.
D) A squirrel collects and stores acorns and nuts in a tree for winter.