cfa一级的derivatve中有一道课后题不是很明白，烦请各位大神指点一下，非常感谢！
题目问derivatives pricing models use the risk-free rate to discount future cash flows 的原因是什么。选项有三个：A. these models are based on portfolios with certain payoffs
B. these models assume that derivatives investors are risk-neutral
C. these models assume that risk can be eliminated by diversification
答案是A  但是我觉得是 B