When she was young,she wanted to go to school.But she could

只能选A afford .for her 中的for 是'为'的意思.pay...for中for后接的宾语应是花钱所购的对象.
如果用pay...for 的话,应是：When she was young,she wanted to go to school.But she could not,because her family was too poor to pay for the school fees for her.