问题如下：
Can discount factors ever be an increasing function of maturity? Discuss.
选项：
解释：
Discount factors normally decrease as the time to maturity increases. An exception would be when interest rates are negative. Since the 2007–2008 crisis interest rates have been negative in several countries.
老师你好，这道题的思路是yield curve shape 里 spot curve 通常情况是 normal，以及 discount rate和spot rate 是倒数的关系 吗？
