To survive in the intense trade competition between countries, we must______ the qualities and varieties of products we make to the world-market demand.

A．Migration of large numbers of people may cause conflict.
B．The newest arrivals can hardly get good jobs.
C．Native people often resent newest arrivals because they cannot communicate easily with the latter.
D．Usually it has taken several decades for each group of newest arrivals to be accepted into the mainstream of society in the host country.

A．How biochemical changes occur in the human brain.
B．Why people should keep active not only physically but also mentally.
C．How intellectual activities influence brain-cell health.
D．Why people should receive special mental training as they age.