[求助]gwd-17-21

Q21:

Medicare, the United States government’s health insurance program for the elderly and disabled, covers the full cost of home health care, but not with other nonhospital services where 20 percent of the costs must be paid by beneficiaries.

1. but not with other nonhospital services where 20 percent of the costs must be paid by beneficiaries
           
2. but not of other nonhospital services, making beneficiaries pay 20 percent of the costs
           
3. but not of other nonhospital services, for which beneficiaries must pay 20 percent of the costs
           
4. which is unlike other nonhospital services in that 20 percent of the costs must be paid by beneficiaries
           
5. which is unlike other nonhospital services that make beneficiaries pay 20 percent of the costs

key:C

偶想问问答案c中for前面的逗号作何用？像这种介词+which 的结构不是紧跟其所修饰名词后面的吗？麻烦各位赐教了~~

谢谢解释，但我不明白for 前面为什么要加逗号

答案B为什么不能理解成前面一句的一个结构用分词表达？