According to Fozard's argument, people can make their brain work more efficiently by______ .

A．How biochemical changes occur in the human brain.
B．Why biochemical changes Occur in the human brain.
C．How intellectual activities influence brain-cell health.
D．Why people should keep active not only physically but also mentally.

A．keep fit by going in for physical activities
B．keep mentally active by challenging their brains
C．maintain mental alertness through specific training
D．maintain a balance between individual and group activities