正确答案: B

盐水和铜

题目：下面各组材料，均为导体的是【】

解析：B

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举一反三的答案和解析：

[单选题]B

Spending two or three hours playing outdoors each day can reduce a child’s chance of becoming short-sighted, a research shows. It challenges (挑战) the belief that short- sightedness is caused by computer use, watching TV or reading in weak light. The Australian government researchers believe that sunlight is good for people’s eyes. They compared the vision(视力) and habits of 100 seven-year-old children in Singapore and Australia. In all, 30% of the Singaporean children were short-sighted--this rate(比率) was ten times higher than Australian children. Both groups spent a similar amount of time reading, watching television and playing computer games. However, the Australian children spent an average(平均) of two hours a day outdoors—90 minutes more than the Singaporean children. Professor(教授) Ian Morgan, from the Australian Research Council’s Vision Centre, said, “Humans are naturally long-sighted, but when people begin to go to school and spend little or no time outdoors, the number of short-sighted people gets larger. We’re also seeing more and more short-sighted children in cities all around the world—and the main reason may be that city children spend less time outdoors. “ Daylight can be hundreds of times brighter than indoor light. But why does playing outside prevent us from becoming short-sighted? Scientists believe that natural light has a special chemical(化学物质) which stops the eyeball from growing out of shape and prevents people becoming short-sighted. So be outdoors. It doesn’t matter if that time is spent having a picnic or playing sports. 24. How much time did the Singaporean children spend outdoors on average every day in the research?

30 minutes.

[单选题]27． From the letter we can conclude(得出结论) that ．

Alice didn't get along well with her classmates

解析：27.C【解析】根据文章的第2～4段可知，Alice与同学相处得不好。

[单选题]Nowadays science fiction isn’t as ___________ as cartoons among teenagers．

popular

[单选题]_______

Before

[单选题]46． Every week， we have classes ________ Monday to Friday．

from

解析：46．C【解析】from…to…意为“从……到……”

[单选题]34．—It’s difficult for the village children to cross the river for school．— I think a bridge ___________over the river．

should be built

解析：34．D【解析】句意；——对农村的孩子们来说，穿过一条小河去上学实在是太难了。——我想应该在河上建一座桥。上句中陈述了一个困难，用“should”提出建议，表示应该怎么办。故选D。