问题如下：
The dependence structure between the returns of financial assets plays an important role in risk measurement. For liquid markets, which of the following statements is incorrect?
选项：
A. Correlation is a valid measure of dependence
between random variables for only certain types of return distributions.
B. Even if the return distributions of two
assets have a correlation of zero, the returns of these assets are not
necessarily independent.
C. Copulas make it possible to model marginal
distributions and the dependence structure separately.
D. Correlation estimates based on short
lookback horizons (three months or less) are typically very stable.
解释：
D is correct.
考点Copula Functions
解析：D选项涉及correlation有关的结论，correlation在经济好的时候比较低，在经济差的时候会增加，所以短期的相关性系数是不稳定的。
麻烦老师解释一下C选项什么意思？
