That Social Security payments will be a burden com...

C

- Hướng dẫn giải

Phương pháp giải:

Giải chi tiết:

Đáp án là C. Trong câu: “Recently, there has been concern that the Social Security fund may not have enough money to fulfill its obligations in the 21st century (=Social Security payments will be a burden) , when the population of elderly Americans is expected to increase dramatically (= the number of elderly people is growing).