Your cousin has a 5-year-old son and a 3-year-old daughter. He has been taking them to story time at the library, but his daughter is having a very difficult time sitting still, even for 10 minutes. His son is able to sit through the entire story time because his ______________ is more fully developed than his sister's.

a. reticular formation
b. hippocampus
c. corpus callosum
d. cerebellum

Answer: A