What Does It Mean to Say That the Government Pension

What does it mean to say that the government pension system is a pay-as-you-go system?

A) Present retirees are being paid from the contributions they have made themselves.
B) Present retirees are being paid from the future contributions of later generations.
C) Future retirees will be paid from the future contributions of later generations.
D) Future retirees will be paid from the contributions of the present generation.