一堆沙子有a吨，每天运走6吨，3天后还剩下（）吨。

根据对话内容，在每小题的空白处填入适当的话语，全对话意思完整。

(Betty is talking with her friend Daming on the phone.)

Daming: Hello, this is Daming. Who’s that?

Betty: This is Betty. I want to visit your hometown—Tianjin. Can you give me some suggestions?

Daming:          1          . What do you want to know?

Betty:             2           ?

Daming: Yes, you are right. it’s usually very hot in summer.

Betty:             3           ?

Daming: I think you’d better come in October.

Betty:                  4          ?

Daming: Because the weather starts go get cooler and the trees start to change colour.

Betty:              5              ?

Daming: You can visit many places of interest here, such as Tianjin Ancient Culture Street and you can also try some traditional Chinese food.

Betty: I’m looking forward to visiting Tianjin. Thank you for your suggestions. Bye-bye.

Daming: bye-bye. I hope to see you soon.

“人类基因组计划”于（）启动，有美国.英国.法国等（）个国家的科学家共同参与。

如图8所示，一金属直杆MN两端接有导线，悬挂于线圈上方，MN与线圈轴线处于同一竖直平面内，为使MN垂直纸面向外运动，可以       (　　)

A．将a、c端接电源正极，b、d端接电源负极

B．将b、d端接电源正极，a、c端接电源负极

C．将a、d端接电源正极，b、c端接电源负极

D．将a、c端接交流电源的一端，b、d端接交流电源的另一端

我国甲药品批发企业代理了境外乙制药厂商生产的疫苗，销售使用后，发现该疫苗存在安全隐患，应实施召回，该药品召回行为的主体应是（）

A.乙制药厂商

B.疫苗销售地省级药品监督管理部门

C.甲药品批发企业所在地省级药品监督管理部门

D.甲药品批发企业

设X₁，X₂，…，X_n(n＞2)为来自总体N(0，1)的简单随机样本，

为样本均值，记Y_i=X_i-

=1，2，…，n．
求：(1)Y_i的方差DY_i，i=1，2，…．n；
(2)Y₁与Y₂的协方差Cov(Y₁，Y_n)．