One reason why brain functioning begins to change in middle adulthood is that

a) middle-aged adults do not take good care of themselves, and it affects the health of their brains.
b) middle-aged adults carry greater loads of stress, which adversely affects their health.
c) researchers have found that 20 genes that are vital to learning, memory, and mental flexibility begin to function less efficiently as early as age 40.
d) middle-aged adults are too busy to participate in mental learning and activities to maintain mental flexibility.

c