The key developments of the brain in the first two years include the

A) personal contact cerebellum.
B) environment and genetics.
C) myelin sheath and dendrite connections.
D) None of these.

The key developments of the brain in the first two years include the growth of the myelin sheath and the formation of dendrite connections, which allow for efficient communication within the brain. These developments are critical for cognitive and motor development, and they are shaped by both genetics and environmental factors. There is no mention of a "personal contact cerebellum," so option A is incorrect. Option B is partially correct but not specific enough, as it does not mention the specific neurological developments. Therefore, the best choice is C.