请问老师第13题A，D选项怎么不对，该怎么改

请问老师第11题B选项为什么不对，该怎么理解

请问老师第四题题目和选项是啥意思，没看懂

原版书原文Settlement risk is the risk due to the exchange of cash flows when a transaction is settled. Failure to perform on settlement can be caused by a counterparty default, liquidity constraints, or operational issues. This risk is greatest when payments occur in different time zones.为什么说这种风险不同时区还款时最大呢？两者有什么联系吗？

原版书上这段：Downgrade risk is the risk that the perceived creditwor-thiness of the borrower or counterparty might deteriorate. In general, deteriorated creditworthiness translates into a downgrade action by the rating agencies, such as Standard and Poor's (S&P), Moody's, or Fitch in the United States, and an increase in the risk premium, or credit spread of the borrower. 这里最后一句的意思不就是说credit spread的增加么？那这个是信用风险啊。为什么习题里面有一个credit spread widen following recent bankruptcies要归属为市场风险呢？

option-adjusted duration是哪里的知识点，能解释一下吗

15题不太明白，麻烦老师。

这道题不应该是价格的波动率吗？不应该价格只差的平方×概率，做和吗？答案求解的是收益率的波动率呢？

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老师，麻烦问下384题，为什么current USD/AUD也要折现呢？