数据库中，从两个关系的笛卡尔积中选择属性间满足一定条件的元组，这种操作是（）A．选择

年幼的留守儿童最需要的是生活照顾，满足他们营养和健康的需要，发展与人相处的社会能力以及接受学前教育等。此时，建立一些农村幼儿园由受过专业训练的幼儿老师和社会工作者来为他们提供服务这一举措符合选择介入行动中的（）原则。

A.以人为本、服务对象自决

B.考虑服务对象的发展阶段和他们的特点

C.瞄准服务目标、考虑经济效益

D.与服务对象相互依赖

As people age, the number of calories they need each day decreases while their daily requirement of vitamin B₆ increases. Clearly, unless older people take 136 supplements or eat a diet that contains more 136 than did the diet they ate when they were young adults, there is little likelihood that they will get B₆ in needed amounts.
Which of the following would be most helpful to know to evaluate the argument

A．Whether the relative decrease in the daily requirement of calories is greater than the relative increase in the daily requirement of vitamin B₆.

B．Whether the form of vitamin B₆ found in dietary supplements is more readily absorbed and utilized by the body than is the from of this vitamin that is found in food.

C．Whether the consequences of not getting vitamin B₆ in required daily amounts are more serious for older people than for young adults.

D．Whether the diets of most people, when they are young adults, include vitamin B₆ in amounts far in excess of their daily needs.

E．(E) Whether the diets of older people are more likely than those of young adults to include one or more meals a day that are devoid of foods containing vitamin B₆.

根据产生数据的特征，将数据划分若干组的方法是______。

A．直方图表

B．调查表

C．分层法

D．聚类法

访问控制的概念是（）。

A.是针对越权使用资源的防御措施

B.针对企业访问用户的管理

C.针对黑客入侵的防御措施

D.针对外部访问权限的管理

某工程队有建安工人300人，月计划完成施工产值3750000元，百元产值工资系数14%，试计算计划工资总额为（）

A.520000

B.525000

C.530000

D.535000