__________是世界上海外华人分布最集中的地区。

地球上最大的生态系统是（　　）

A．草原生态系统

B．森林生态系统

C．海洋生态系统

D．生物圈

胃肠道平滑肌动作电位的产生，主要依赖于

A．Na⁺和Ca²⁺内流

B．Ca²⁺内流

C．Na⁺内流

D．K⁺内流

Homelessness is a huge problem all over the world. It’s said that each day there are about 1 million people living on the streets in the US. It is a serious problem which should be dealt with effectively.

There are two main reasons for people leaving their homes: poverty and less affordable houses due to bad economic conditions. Other reasons include: domestic violence, divorce, and mental illness. Living without a home makes people main targets of violence activities and besides, these homeless people have to face the following difficulties on a daily basis.

They have to move about, carrying their belongings, which makes it difficult for the charity and the government to help them. They have limited access to bathrooms and restrooms and are forced to live in dirty conditions, making them easy to be attacked by diseases. They have limited access to health-care facilities in case of emergency, and almost no access to education as they have no money. They are often ill-treated by society, which makes them unfriendly, often ending up being involved in activities connected with crime like selling drugs or guns.

Homelessness is not something that happened overnight --- it has been in the country for decades. The society usually believes those living on the street to be mentally unstable. This belief is partially true. Most of them are normal people just like you and me.

The federal government is doing all it can to help but it is not enough, so the ordinary citizens have to stand up and try to help whenever it’s possible. There are various organizations, such as charity organizations that are working constantly to make sure that homeless people find a shelter to live in. the united efforts have helped millions of people return to their normal lifestyle. However, there is still a long way to go before a better situation is created for all the homeless.

小题1:Homeless people suffer from all the following except __________.

A．violent activities

B．disease attack

C．moving a lot

D．heavy work小题2:The homeless are usually treated as __________.

A．having mental problems

B．a danger to society

C．physically disabled people

D．being too lazy to do anything小题3:Which of the following are mentioned to help solve the problem of the homeless?

① the government ② homeless people ③ charity organizations ④ the ordinary citizens

A．①②③

B．①③④

C．①②④

D．②③④小题4:What can we learn from the passage?

A．Most homeless people died of hunger and cold.

B．The homeless should work hard to change their own situation.

C．The homeless problem is a serious problem that need solving effectively.

D．The homeless people have nothing to take when they move about.

机体抵抗病原微生物感染的功能称为（）。

A.免疫监视

B.免疫自稳

C.免疫防御

D.免疫识别

E.免疫耐受

空间有一直线，在H和W的投影均为直线且平行于Y轴，则该直线为（）。

A.正垂线

B.铅垂线

C.侧垂线

D.都不是