问题如下：
What are beta measures?
选项：
A. The volatility of the security
B. The joint volatility of any two securities in a portfolio
C. The volatility of a security divided by the volatility of
the market index
D. The relative co-movement of a security with the market portfolio
解释：
D.
The relative co-movement of a security with the market portfolio
老师您好，请问可以解释一下C和D的意思吗？谢谢