Based on this chart, why is the Social Security Administration facing a funding problem?

If the choices are:

A) Life expectancy is increasing slower than expected, so people are not living as long as thought. B) Younger people are no longer paying as many retirement taxes as in past generations. C) The number of elderly Americans is growing quickly, so more money is going out than coming in. D) An unequal amount of taxes is being allocated to Medicare and Medicaid services.

Then then answer is " C) The number of elderly Americans is growing quickly, so more money is going out than coming in."