问题如下：
Which of the following operations can make the capital allocation line to have higher rates of return for given risk levels greater than the optimal risky portfolio on efficient frontier?
选项：
A.Lending at the risk-free rate.
B.Borrowing at the risk-free rate.
C.Purchasing the risk-free asset.
解释：
B is correct.
The ability of the investor to borrow at risk free rate to purchase additional amounts of the optimal risky portfolio makes higher rates of return for levels of risk greater than the optimal risky assets.
答案选择对了，但是还是需要解释一下题目和答案
