甲-（乙+丙）=（ ） A．甲-乙+丙 B．甲-乙-丙 C．甲-丙+乙

阅读理解。

     Have you ever wondered why birds sing? Maybe you thought that they were just happy. After all,

you probably sing or whistle when you are happy.

     Some scientists believe that birds do sing some of the time just because they are happy. However,

they sing most of the time for a very different reason. Their singing is actually a warning to other bird

s to stay out of their territory.

     Do you know what a "territory" is? A territory is an area that an animal, usually the male, claims as

its own. Only he and his family are welcome there. No other families of the same species are welcome.

Your yard and house are your territory where only your family and friends are welcome. If a stranger

should enter your territory and threaten you, you might shout. Probably this would be enough to frighten

him away.

     If so, you have actually scared the stranger away without having to fight him. A bird does the same

thing. But he expects an outsider almost any time, especially at nesting season. So he is screaming all

the time, whether he can see an outsider or not. This screaming is what we call a bird's song, and it is

usually enough to keep an outsider away.

     Birds sing loudest in the spring when they are trying to attract a mate and warn others not to enter

the territory of theirs.

     You can see that birds have a language all their own. Most of it has to do with attracting mates and

setting up territories.

1. According to the passage, most of the time birds' singing is actually        .

A. an expression of happiness

B. a way of warning

C. an expression of anger

D. a way of greeting

2. What is a bird's "territory"?

A. A place where families of other species are not accepted.

B. A place where a bird may shout at the top of its voice.

C. An area for which birds fight against each other.

D. An area which a bird considers to be its own.

3. Why do birds keep on singing at nesting season?

A. Because they want to invite more friends.

B. Because their singing helps frighten outsiders away.

C. Because they want to find outsiders around.

D. Because their singing helps get rid of their fears.

4. How does the writer explain birds' singing?    

A. By comparing birds with human beings.

B. By reporting experiment results.

C. By describing birds' daily life.        

D. By telling a bird's story.

手摸钨棒后应注意什么？

某建设单位与某施工单位订立了施工合同，建设单位负责电梯设备的采购。于是建设单位向电梯生产厂家发函要求购买两部电梯。电梯厂回函表示“其中一部可以按要求期限交付，另一部则需延期10日方能交付”。电梯厂的回函属于( )。

A．要约邀请

B．要约撤销

C．承诺

D．新要约

现有一个员工基本情况表，有编号、姓名、性别、出生年月、入职年份、职务等字段，能作为主关键字的是（）。

A、姓名

B、编号

C、出生年月

D、职务

外部公众（）

A、不是组织外部的人群结合体

B、是独立于组织之外的组织或群体

C、与内部公众相比，数量更小

D、与内部公众相比，层次种类更简单