According to the passage the average I.Q. is _________.

A．individuals with identical brains seldom test at the same level
B．an individual's intelligence is determined solely by his environment
C．children reared under average conditions possess average intelligence
D．lack of opportunity hinders the growth of intelligence

A．human brains differ considerably
B．the brain a person is born with is important in determining his intelligence
C．environment is crucial in determining a person's intelligence
D．persons having identical brains will have roughly the same intelligence