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

如何理解这道题解析中的the concave/convex efficient frontier 在CML，没有加入risk-free asset之前efficient frontier不都是convex的吗

老师，在推导asset or nothing call时，杨老师说asset和N(d2)有相关性，这怎么理解？

这个结论我觉得有问题，比如说long asset …… short cash……时，得出一个call,MAX(St-Q), Q我可以理解成K，因为本来就是固定的一个现金值，可是那个St就是S其实是资产啊，怎么就能说成是个call了，call的St是股票价格，是会变动的。

老师，我听不懂关于CHOOSE OPTION里分拆两个期权的做法，为什么CALL就说是到期时间T，而PUT就是到期时间是t？

如何理解倒数第二句话中的if the true coverage is 97% of exceptions instead of the required 99%

15题不太明白，麻烦老师。