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
参考答案：D
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| A lady and her husband stepped off the train in Boston. They walked without an appointment (预约) into the outer 1 of Harvard's president. But they were 2 by his secretary and kept waiting. For hours, the secretary took no notice of them, 3 that the couple would finally become 4 and go away. But they didn't. The secretary finally decided to disturb the president, though 5 . A few minutes later, the president walked towards the couple with a 6 face. The lady told him, "We had a son that 7 Harvard for one year. He loved Harvard. He was 8 here. But about a year ago, he was accidentally killed. My husband and I would like to 9 a memorial (纪念物) to him, somewhere on campus." The president wasn't 10 . Instead, he was shocked. "Madam," he said, "we can't put up a statue for every person who studied at Harvard and died. If we did, this 11 would look like a cemetery (墓地)," "Oh, no," the lady 12 quickly. "We don't want to put up a statue. We would like to give a 13 to Harvard." The president rolled his eyes and 14 at the couple and then exclaimed," A building! Do you have any 15 how much a building costs? We have spent over ＄7,500,000 on the campus building at Harvard. " For a moment the lady was silent. The president was 16 , because he could get rid of them now. Then the lady turned to her husband and said quietly,"Is that all it costs to start a 17 ? Why don't we just start our own?" Her husband nodded. 18 their offer was turned down. Mr. and Mrs. Stanford traveled to California where they founded the University that bears their 19 , a memorial to a son that Harvard no longer 20 about. | ||||
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sin
V