In What Sense Is Social Security Spending "Uncontrollable

In what sense is Social Security spending "uncontrollable"?

A) Congress determines how much it will spend for Social Security each year.
B) Social Security spending varies according to how much money is available in the Social Security Trust Fund.
C) Congress has enacted laws that make Social Security payments automatic for all those who are eligible.
D) Social Security requires funding from excise taxes that are unpredictable from year to year.