“How much shall I pay for the phone call?”  “You________. I

“How much shall I pay for the phone call?”  “You________. It’s free of charge.”
A、shouldn’t
B、can’t
C、don’t have to
D、mustn’t
【正确答案】：C
【题目解析】：由后面的This is free of Charge。可以知道答案，you don't have to 你不必这么做。