轨道车定期保养有什么要求？

假定经济处于完全就业状态，总供给曲线为古典的情形，即总供给曲线垂直时，名义货币供应量增加20%，则______。

A．对价格没有任何影响
B．利率上升
C．名义工资上升20%
D．实际货币供给上升20%

某溶液中含有Cl^-、SO₄^2-、Fe³⁺、K⁺、M，且离子物质的量之比为2∶3∶1∶3∶2，则微粒M可能为  

A．Na⁺

B．Ag⁺

C．Ba²⁺

D．Mg²⁺

釉牙骨质界中最常见的情况是

A．少量牙骨质覆盖在釉质表面

B．釉质与牙骨质端口相接

C．牙釉质覆盖牙骨质

D．釉质和牙骨质分离

E．以上都不是

(D)

You can not see any object unless light from that object gets into your eyes. Some of the things you see give off light of their own. The sun, the stars, a lighted lamp are examples that can be seen by their own light. Such things are luminous. Most of the things you see are not giving off light of their own. They are simply reflecting light that falls on them from the sun or some other luminous bodies. The moon, for example, does not give off any light of its own. It is non-luminous. You see it because sunlight falls on it and some of it reflects in our direction. So moon light is only second hand sunlight.

When you look at a book, it sends to your eyes some of the light which falls on it, and you see the book. If light could be kept out from where you are so that there would be no light for the book to reflect, then you could not see the book even with your eyes wide open.

Light travels so fast that the time in which it travels from the book you are reading to your eyes is so short as if there were no time at all. Light reaches us from the moon, which is about 380 000 kilometers away, in only a little more than a second.

小题1:You can see the book because_______.

A．your eyes are close to it

B．it reflects some of the sunlight

C．it has light of its own

D．your eyesight can get to it小题2:The underlined word “luminous” means_______.

A．visible

B．all colors

C．giving off light

D．sunlight小题3:________ have light of their own.

A．The sun and the moon

B．The stars and the earth

C．The sun and the stars

D．The moon and the earth小题4:Which of the following is true?

A．All the things you can see give off light.

B．Light from the book is much shorter than that from the moon.

C．The moment you open your eyes the light from the book travels to your eyes.

D．Light travels so fast that there is no time for you to read.小题5:This article is probably from _________ .

A．a science magazine

B．a medical magazine

C．a biography

D．a story book.

科教兴国最重要的环节是

A．普及义务教育

B．基础教育

C．高等教育

D．扫除文盲