Gray matter in the prefrontal cortex decreases during adolescence. Why is this seen as a positive change? 

a) Synaptic pruning removes unused synapses, fine-tuning the brain. 

b) Some gray matter neurons were defective and needed to be removed. 

c) The gray matter gets converted to white matter as myelin is removed. 

d) Neurons migrate from the prefrontal cortex to other areas of the cortex, strengthening other lobes.