问题如下：
By which degree can a portfolio affect the risk and return proflie?
选项：
A. Portfolios equally affect risk reduction and return enhancement.
B. Portfolios affect more on risk reduction.
C. Portfolios affect more on return enhancement.
解释：
B is correct.
Portfolios affect more on risk reduction.
A 是不是错在不能确定对return的enhance结果