Answer:
A, a challenge for Medicare and Social Security
Explanation:
An aging population is a problem many developed countries face today. It occurs because of the demographic situation today: low or negative birth rates.
An aging population means that there are more retired than working people. The retired people don't pay taxes and many rely on governmental medical and social programs such as Medicare and Social Security. These programs have to be financed from public funds and the absence of taxes means there may be not enough money to cover for the expenses in the future.
