正确答案: B

5%

题目：将50g20%氯化钾溶液稀释到200g，稀释后所得溶液中溶质的质量分数是（ ）

解析：B

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举一反三的答案和解析：

[单选题]_______________.

outside

[单选题]27． Jack didn't like to ________ when he was a student．

do his homework

解析：27.B【解析】由第一段第三句可知。

[单选题]2． I see __________ alarm clock on the table．

an

解析：2．B【解析】以元音音素开头的可数名词，前面的冠词用an。

[单选题]下列句子中标点符号使用不恰当的一项是（ ）

“吹面不寒杨柳风，”不错的，像母亲的手抚摸着你。

[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.