Most of the growth in the brain during middle childhood is from

A) growth in the blood vessels that spread through the brain.
B) increases in glial cells.
C) continued myelination of neurons.
D) all of the above

D) all of the above. Most of the gain in brain growth is due to growth in the blood vessels that spread through the brain, a gain in the number of glial cells that support the function of neurons, and continued myelination of the axons that make up the brain's circuits.