Which of the following sentences is courteous?()

Which of the following sentences is courteous？()

A.Am I the only one who can read the operating manual？

B.If you had listened to our agent more carefully, you would know that your policy does not cover accidents outside the UK.

C.We are sorry you have misunderstood us.

D.Your letter of June 3rd regarding payment of this order has received our careful attention.

请帮忙给出正确答案和分析，谢谢！