Given a scientific question, generate a correct answer to it.

[EX Q]: When does diseases like alzheimer's become more common?
[EX A]: old age

[EX Q]: How long do the long-lived isotopes require to decay to a safe level in a nuclear reactor?
[EX A]: thousands of years

[EX Q]: What is defined as a saturated layer of rock or soil?
[EX A]:
aquifer