Eight-Year-Old Ella Can Use Scissors to Cut Small Paper Dolls

Eight-year-old Ella can use scissors to cut small paper dolls out of craft paper, something she could not do at age three. What best accounts for her improving dexterity?

A) increased cortical thickening in the temporal lobe
B) increased myelination of the central nervous system
C) increased bone ossification
D) increased muscle development