A．They fought with the other mice.
B．They stayed close to their mothers.
C．They ran back and forth constantly.
D．They remained close to one wall.

A．The extent of damage to the nervous system.
B．The presence or absence of certain nerve-cell receptors.
C．The size of nerve-cell receptors in the brain.
D．The level of danger in the mammal's environment.

A．To show the relationship between fearfulness and environment.
B．To give examples of animals that they aren't fearful.
C．To compare fear in mammals to fear in other animals.
D．To identify the nerves that control fear in certain animals.