说明核苷酸降解的一般途径，嘌呤与嘧啶的降解有何区别？

根据文意，对“太阳能发电卫星”下定义最为准确的一项是（）。
阅读下面短文，完成下面4个小题
所谓太阳能发电卫星是指载太阳能电池壁板的卫星。它在轨道上保持与地球同步时，所产生的电能交换成微波束送回地面使用。日本科学家设想的太阳能发电卫星SPS2000大致上由两个系统组成。一是在宇宙空间进行发电的卫星，二是在地面上接收从卫星发回来电波的受电设施。
发电卫星的形状为每边336米的正三角柱，柱长为303米。三角柱的两个侧面布满着非晶态硅太阳能电池，发电功率为每平方米1.4千瓦。剩下的朝着地球的这一面的中央，放置着边长为132米的正方形的送电天线。
发电卫星的组成，除发电、集电以及送电部分外，还有起通信、控制作用的总线部分，形成卫星骨架的结构部分，用于组装和维护用的机器人等，总重量约240吨。
接收卫星发回来的微波能束，并将其变换为电能的地面设施称为受电设施。它由负责卫星跟踪和进行受电控制的控制部分，以及负责送电和储电的部分等组成。为了能把卫星发送回来的全部电能都接收下来，需要直径2千米的受电设施。受电设施分布在若干地方，当每次卫星对一个地方的受电设施送电时，可以受电的时间为227秒，而受电的周期为116分左右，可平均连续地发出250千瓦的电能。
由于受电设施非常庞大，所以有人设想采用线网状反射式的受电设施，可以只把电波集中起来，而能从网眼中透过80%以上的阳光，从而不对环境造成影响。
从卫星上向地面受电设施送电的机构，接收从地面上往卫星发送的导向电波束，根据这一电波束，卫星便能计算出受电设施位于什么地方，然后相应地改变电微波的相位，使微波束朝着受电设施方向送电。所以受电设施以外的地方，不用担心高能电微波束的照射。就是在受电部分电能量密度最高处的受电设施中心部分，电能量密度也在每平方厘米1毫瓦以下（每平方米10瓦以下）。完全能够满足在2.45兆赫时国际安全标准的要求，对人体和环境都不会有影响。

A．太阳能发电卫星是搭载太阳电池壁板在宇宙空间进行发电的卫星

B．太阳能发电卫星是与地球轨道保持同步并在宇宙空间进行发电的卫星

C．太阳能发电卫星是搭载太阳电池壁板并将电力变换成微波束送回地面使用的卫星

D．太阳能发电卫星是由在宇宙空间进行发电的卫星与在地面上接收电波的受电设施组成的

One morning, a blind boy sat beside a building with a hat by his feet. He held a sign which said, “I am blind. Please help me.”

There were only a few coins in the hat. A man was walking by. He took out a few coins from his pocket and dropped them into the hat. He then took the sign, turned it around, and wrote some words on it. He put the sign back so that everyone who walked by could see the new words.

Soon the hat began to fill up. A lot more people were giving money to the blind boy. That afternoon the man who had changed the sign came to see how things were. The boy recognizedhis footsteps and asked, “Are you the one who changed my sign in the morning? What did you write?”

The man said, “I only wrote the truth. I said what you said, but in a different way.”

What he had written was, “Today is a beautiful day, but I can’t see it.”

Do you think the first sign and the second sign were saying the same thing?

Of course both signs told people the boy was blind. But the first sign simply told people to help by putting some money in the hat. The second sign told people that they were able to enjoy the beautiful day, but the boy could not enjoy it because he was blind.

The first sign simply said the boy was blind, while the second sign told people they were so lucky that they were not blind.

There are at least two lessons we can learn from this simple story.

The first is: Treasure (珍惜) what you have. Someone else has less. Try your best to help those who need your help.

The second is: Be creative. Think differently. There is always a better way.

小题1:From the article we know that________.

A．the boy always has a hat on his head

B．the man is one of his neighbors

C．the boy is blind and needs help

D．the boy works for the man小题2:On the sign what the man wrote was, “                .”

A．Treasure what we have already had

B．Today is a beautiful day, but I can’t see it

C．We are so lucky that we are not blind

D．Try your best to help those who need your help.小题3:According to the article, which of the following is TRUE?

A．The hat began to fill up after the man wrote some words on the sign.

B．The man took away all the few coins from the hat.

C．The boy wanted to get enough money to go to school.

D．Nobody wanted to give any coins to the blind boy.小题4:The word “recognized” here in Chinese means _______.

A．承认

B．接受

C．表扬

D．认出小题5:_______ is the best title (题目) for this story.

A．Be careful not to be blind.

B．Be creative. There is always a better way!

C．Be kind to the blind!

D．Don’t always get, but offer!

新购进的紫外线灯管其辐照强度不得低于（）。

A．70μW/cm²

B．80μW/cm²

C．90μW/cm²

D．100μW/cm²

E．180μW/cm²

牙槽骨吸收的速度与那种因素无关（）

A.缺牙时间

B.缺牙原因

C.骨的致密度

D.患者的年龄

E.全身因素

试述电力机车车轴的受力情况。