问题 单项选择题 共用题干题

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。

关于滤线栅使用注意事项的叙述,错误的是()

A.将滤线栅置于焦点和被照体之间

B.焦点到滤线栅的距离与栅焦距相等

C.X线中心线对准滤线栅的中心

D.原投射方向与滤线棚铅条排列间隙平行

E.原发X线与滤线栅铅条平行

答案

参考答案:A

阅读理解

阅读理解

     It has been proved that some children are natural--born bosses. They have a strong need to make

decisions, manage their environment, and rather than follow. Stephen Jackson, a Year One student,

operates under the theory of what's mine is mine and what's yours is mine. His mother says: "The other

day I bought two new Star Wars light sword. Later, I saw Stephen with the two new ones while his

brother was using the beat-up ones."

     "Examine the extended family, and you will probably find a bossy grandparent, aunt, uncle or cousin

in every generation. It is an inheritable trait." says Russell Barkley, a professor at the Medical University

of South Carolina. Other children who may not be particularly bossy can gradually gain dominance(支配地位) when they sense their parents are weak, hesitant, or in disagreement with each other.

     Whether it is inborn nature or developed character at work, too much control in the hands of the

young isn't healthy for children or the family. "Fear is at the root of a lot of bossy behavior," says family

psychologist John Taylor in his book From Defiance to Cooperation, "and children have secret feelings

of weakness and a desire to feel safe. It is the parents' role to provide that protection."

     When a "bossy child" doesn't learn limits at home, the stage is set for a lot of troubles outside the

family. The overly willful and unbending child may have trouble obeying teachers or coaches, for example, keeping friends. It can be pretty lonely as the top dog if no one likes your bossy ways.

     "I see more and more parents giving up their power, " says Barkley, who has studied bossy behavior

for more than 30 years. "They bend too far because they don't want to be as strict as their own parents

were. But they also feel less confident about their parenting skills. Their kids, in turn, feel more anxious."

1. Bossy children like Stephen Jackson           .

A. make good decisions

B. show self-centeredness

C. Lack care from others

D. have little sense of fear

2. The underlined phrase "beat-up " in paragraph 1 means             .

A. old

B. bad

C. pretty

D. moved

3. The study on bossy behavior implies that parents should            .

A. give more power to their children

B. not be so anxious about their children

C.  be strict with their children

D. not set limits for their children

4. Bossy children may probably become                .

A. relaxed

B. skillful

C. hesitant

D. lonely

单项选择题 A1/A2型题