问题如下：
Investors should use a portfolio approach to:
选项：
A.reduce risk.
B.monitor risk.
C.eliminate risk.
解释：
A is correct.
Combining assets into a portfolio should reduce the portfolio’s volatility. Specifically, "individuals and institutions should hold portfolios to reduce risk." As illustrated in the reading, however, risk reduction may not be as great during a period of dramatic economic change.
为什么是reduce不是monitor？
老师上课的时候不是提及不同的人有不同的indifference curve，那么，也就是说，有的人会选择pofolio with higher risk，比如有的是borrowing 有的是lending ?
