Speaker A: Did she really use to be very thin？ Speaker B: ______A．How do you know that？B．Y

Speaker A: Did she really use to be very thin？ Speaker B: ______

A．How do you know that？

B．Yes, she did, but she isn"t any more.

C．I can"t believe what you"ve said.

D．I"m afraid that you are totally wrong.

请帮忙给出正确答案和分析，谢谢！