问题 单项选择题 A3/A4型题

滤线栅是由许多薄的铅条和易透过X线的低密度物质作为充填物,交替排列组成的。在X线摄影中使X线的中心线对准滤线栅板中心,原射线投射方向与滤线栅的铅条排列间隙在同一方向上,能通过铅条间隙而到达胶片产生影像。由于被照体产生的散射线是多中心、多方向的,其中大部分散射线被铅条所吸收,只有一小部分通过。滤线栅的指标:①栅比(R):滤线栅铅条高度与填充物幅度的比值为栅比。②栅密度(n):n表示在滤线栅表面上单位距离(1cm)内,铅条与其间距形成的线对数,常用线/cm表示。

关于滤线栅栅比的叙述正确的是()

A.栅比越大,透过的散射线越少

B.栅比为铅条宽度与其高度之比

C.栅比为铅条高度与其宽度之比

D.栅比为单位距离内铅条的数目

E.栅比表示单位体积中铅的重量大

答案

参考答案:A

阅读理解

阅读理解

       When I was a child, I often dreamed of the time when I could leave home and escape to the city. We

lived on a farm, in the winter especially, we were quite cut off from the outside world. As soon as I left

school, I packed my bags and moved to the capital. However, I soon discovered that my life has its

problems too.

       One big disadvantage is money. It costs so much to go out, not to mention basics like food and

housing. Another disadvantage is pollution. I suffer from asthma(哮喘), and at times the air is so bad that

I am afraid to go outside. Then there is the problem of traveling round. Although I have a car, I seldom

use it because of the traffic jams. One choice is to go by bicycle, but that can be quite dangerous.

       Of course there are advantages. First, there is so much to do in the city, whatever you tastes in culture or entertainment(娱乐活动). Besides, there are wonderful jobs and greater chances of moving to a more

important job or position. Finally, if you like shopping, the variety of goods is very surprising-and, what is more, shops are often only a short walk away.

       Is life better then, in the city? Perhaps it is, when you are in your teens(十几岁)or twenties. However, as you get older, and especially if you have small children, the peace of the countryside may seem

preferable. I certainly hope to move back there soon.

1. What was the writer always thinking about when he was a child?

A. Staying on the farm.

B. Moving to the countryside.

C. Leaving home for the city.

D. Running away from the school.

2. Which of the following is true about the writer?

A. He is very old now.

B. He is in good health.

C. He prefers driving a car.

D. He lives in the city now.

3. In the passage, the writer tries to __________.

A. express his opinions about way of life

B. describe his life in the countryside

C. show an interest in the outside world

D. persuade the reader to live in the city

4. How is the passage mainly developed?

A. By inferring.

B. By comparing.

C. By listing examples.

D. By giving explanations.

单项选择题 A1/A2型题