【正确答案】 D

【答案解析】______ retire(动词，但又不是谓语) are still able to receive （谓语部分）pension 宾语that they have paid into the social security system during their working days.定于从句。本句缺少主语 a. Most people b. since most people c. Most people are d. Most people who 主语中含有个定语从句most people主语 who retire定语从句