A physically and mentally active
B be alert and receptive
C engage in mental, activities
D the benefits of challenging the brain
E beneficial and happy
F maintain mental alertness
Such specific training as learning to memorize enormous amounts of information is of less interest than being able to_______.

A．They had trouble keeping enzymes in fuel cells active.
B．They had trouble keeping biological cells active.
C．They had trouble producing fresh enzymes.
D．They had trouble finding mechanism for producing enzymes.

A．they
B．it
C．some
D．most