问题如下：
When it comes to the characteristic of commercial paper, we know that:
选项：
A.It is a type of long-term debt.
B.It is secured debt.
C.It is a low rate debt.
解释：
C is correct.
Commercial paper: short term; unsecured; low rate debt.
为啥债券是 unsecured？低利息不就说明风险低，应该是安全的呀？
