og10-sc-64

题目是这样的：

Unlike a funded pension system, in which contributions are invested to pay future beneficiaries, a pay-as-you-go approach is the foundation of Social Security.

我的问题是pay-as-you-go是啥意思啊？

from Longman: 

a pay-as-you-go mobile phone or Internet service is one that you must pay for before you can use it