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
Many experts are putting the theory to work in their own lives because they are convinced of_______.

A．When enzymes can be commonly found in living cells.
B．When the technology of producing a new type of fuel cell appears.
C．When the technology of a new type of fuel cell is suitable for mass production.
D．When the technology of mass producing cell phones appears.