For an investment portfolio, the coefficient of variation of the returns on the portfolio is best described as measuring:（）

A. risk per unit of mean return.

B. mean return per unit of risk.

C. mean excess return per unit of risk.

补出下列名句名篇中的空缺

（1）早岁那知世事艰，中原北望气如山。              ，               。

（2）迢迢牵牛星，皎皎河汉女。                 ，             。

（3）相去复几许？              ，                  。

（4）慨当以慷，忧思难忘。                 ，               。