notes上的一道练习题：

A decrease in the risk-free rate of interest will:

A: increase put & call prices.

B: decrease put prices & increase call prices.

C: increase put prices & decrease call prices.

答案是C，有人可以给个解释吗？risk free interest rate为什么和put price成反向关系和call price成正向关系呢？谢谢啦！