If you are interested, we will send you a sample _______ charge.

If you are interested, we will send you a sample _______ charge.
A、within
B、for
C、with
D、free of
【正确答案】：D
【题目解析】：free of charge 免费。
句意：如果您有兴趣，我们将免费给您寄一份样品。