W公司自2000年创办后一直实行计时工资制，但随着企业规模的扩大，该厂人力资源部门发现员工工作的积极性逐渐下降，向有关专家咨询后，改用计件工资，结果生产率大大提高，但又出现了一些其他问题。根据以上资料，回答下列问题：

该企业停止使用计时工资的原因在于（）。

A.单位产品的人工成本难以确定

B.为防止员工偷懒，需多设监督人员，增加开支

C.报酬与工作时间直接联系，常使报酬与实际产出不一致，缺乏激励机制

D.贡献不同的人报酬基本相同，降低了员工的积极性

Juliet Kaufman manages a large portfolio of risky assets for a family of investors. The portfolio consists primarily of stocks but also includes a small allocation of fixed-income investments. Currently, the expected return and standard deviation of the portfolio are 14.0 percent and 12.0 percent, respectively. Kaufman is considering adding one of four stocks to the portfolio. Data on each stock’s expected return, beta, standard deviation and covariance with the existing portfolio are presented below. Which stock should Kaufman add to the portfolio

A．Stock

B．E(R)

C．Beta

D．D.

E．Covariance

F．

A

B

C

G．

12.0%

13.0%

15.0%

H．

0.85

1.08

0.97

I．

11.0%

10.0%

8.0%

J．

0.00

-0.01

0.02