问题 单项选择题

在X线摄影中,X线束是以焦点作为顶点的锥形放射线束,将被照体G置于焦点与胶片之间时,因为几何投影关系,一般被照体离开焦点一定的距离a(焦-肢距),胶片离开肢体一定的距离b(肢-片距)。所以,肢体在X线胶片上的影像S要比肢体G大,是被放大了的影像,S与G之比即影像的放大率M,而且胶片离肢体越远,影像放得越大。国际放射学界公认:当照片上的半影模糊值<0.2mm时,人眼观察影像毫无模糊之感;当半影模糊值=0.2mm时,人眼观察影像开始有模糊之感。故0.2mm的半影模糊值就是模糊阈值。

关于几何学模糊,叙述错误的是()

A.焦点尺寸越大,影像越模糊

B.应使被照体靠近胶片

C.0.2mm是半影模糊阈值

D.应减小焦-肢距

E.使用小焦点

答案

参考答案:D

选择题
阅读理解

There are two methods by which hydrogen (氢) can be used to power cars. The first way is to use hydrogen to drive the engine, in much the same way as many cars use gas. The second method is to use the reaction of hydrogen with oxygen in a battery, making the car a kind of electric one.

The dream of producing hydrogen in the car while driving along by electrolyzing(电解)water is a long way off, so we are still at the period of batteries and filling the tank with hydrogen gas. This is the difficulty for potential car users and producers. There are only sixteen hydrogen filling stations in Los Angeles and none in 99% of other cities worldwide.

Indeed, some of the big name automobile producers have pulled out of the race to put the first practical hydrogen car on the streets. Ford and GM have announced that they are pulling out in America and so has Renault in France.

However, the Japanese companies are pressing on. In fact, Honda introduced its first hydrogen fuel cell car in 1999. They are now producing second generation hydrogen car known as the FCX Clarity. Guess where they are available for sale? In only one city because of its filling stations.

Honda thinks that they could go into full-scale production of the FCX Clarity by 2020 if the world is prepared for them by then.

Then there are hydrogen-powered buses in several European cities including Amsterdam, Barcelona, Hamburg, London, Luxembourg and Madrid. Lotus, the makers of London taxis, have announced that they propose to manufacture hydrogen-powered taxis in time for the London Olympics.

So, the hydrogen vehicle is out there and the numbers will be growing fairly soon. The buses go back to their bus station, where an electrolyzing machine changes water into fuel for them to fill up on and the same will be the case for many of London’s taxis.

Unfortunately, getting fuel is not the only difficulty for the average motorist, a number of these vehicles cost about $300,000 each.

小题1:According to the text, hydrogen-powered buses ________.

A.can easily be filled up with gas.

B.are likely to sell well in the future.

C.are mainly used in the United States

D.won’t be used in the London Olympics小题2:What does the underlined word “them” in paragraph 5 refer to?

A.Japanese companies

B.Hydrogen buses

C.The FCX Clarity

D.Filling stations小题3:Where are hydrogen cars currently available for sale?

A.In Barcelona

B.In Hamburg

C.In London

D.In Los Angeles小题4:We can learn from the text that ________.

A.there are sixty hydrogen filling stations all over the world

B.hydrogen vehicles fueled by water will be very expensive

C.Honda will mass-produce the FCX Clarity by 2012

D.Honda produced its first hydrogen car in 1989