问题如下：
Which of the following statements comparing VaR with expected shortfall is true?
选项：
A.Expected shortfall is sub-additive while VaR is not.
B.Both VaR and expected shortfall measure the amount of capital an investor can expect to lose over a given time period and are, therefore, interchangeable as risk measures.
C.Both VaR and expected shortfall depend on the assumption of a normal distribution of returns.
D.VaR can vary according to the confidence level selected, but expected shortfall will not.
解释：
A is correct.
考点Coherent Risk Measures
解析VaR measures the expected amount of capital one can expect to lose within a given confidence level over a given period of time. One of the problems with VaR is that it does not provide information about the expected size of the loss beyond the VaR.VaR is often complemented by the expected shortfall, which measures the expected loss conditional on the loss exceeding the VaR. Note that since expected shortfall is based on VaR, changing the confidence level may change both measures. A key difference between the two measures is that VaR is not sub-additive, meaning that the risk of two funds separately may be lower than the risk of a portfolio where the two funds are combined. Violation of the sub-additive assumption is a problem with VaR that does not exist with expected shortfall.
老师好，B选项错误，是不是因为这句话错了？“expected shortfall measure the amount of capital an investor can expect to loss over a given time period”；另外B选项的最后一句“interchangeable as risk measure", 这句话能解释一下吗？
C选项错误是不是因为VaR和ES其实都不一定需要假设normal distribution？
