问题如下：
There are two approaches to calculate risks. In the top-down approach, a bank's portfolio is assumed to be divided into market, credit, and operational risk measures. In the bottom-up approach, interactions among various risk factors are being considered. Different approaches have different assumptions on risk diversification. According to the academic studies which are trying to evaluate the ratio of integrated risks to separate risks, which of the following statements is least accurate?
选项：
A.Top-down studies calculate this ratio to be less than one, which suggests that risk diversification is present and ignored by the separate approach. B.Top-down studies calculate this ratio to be greater than one, which suggests that risk diversification is present and ignored by the separate approach.
C.Bottom-up studies sometimes calculate this ratio to be less than one, but recent findings show evidence of risk compounding, which produces a ratio greater than one.
D.Bottom-up studies suggest that risk diversification should be questioned.
解释：
A is correct.
考点：Top-down approach and Bottom-up approach
解析：
Top-down方法是把三种风险分开考虑的，在每个风险类别里面考虑了相关性。比如说，市场风险里，就考虑了利率和汇率的相关性。因此它的ratio<1，说明多样化是present存在的。
Bottom-up方法是先从每一个风险因子出发，并考虑风险因子间的相关性，最终算出整个组合的VaR。因为考虑了风险因子之间的相关性，所以它考虑到了risk diversification。Bottom-up方法里ratio大多数情况下ratio都是小于1，但有些实证也反映出，buttom-up的ratio会大于1
老师好，能否具体讲一下每一个选项？
