问题 选择题

用显微镜观察洋葱表皮细胞,第一次使用目镜为l0×,物镜为10×:第二次使用目镜为12×,物镜为45×,那么第二次观察到的情况与第一次相比有什么不同?(  )

A.视野亮,细胞数目多

B.视野亮,细胞数目少

C.视野暗,细胞数目多

D.视野暗,细胞数目少

答案

显微镜的放大倍数═物镜放大倍数×目镜放大倍数.第一次显微镜的放大倍数为:10×10═100倍,第二次显微镜的放大倍数为:12×45═900倍,在使用中由低倍镜换成高倍镜后,显微镜的放大倍数增加,视野中细胞的体积变大,细胞的数目减少,视野变暗.

故选:D

阅读理解

阅读理解

     If you go to a fast food restaurant, you will probably see a lot of teenagers. Today, many teenagers

are overweight, and some of this is because of their bad eating habits . Most teenagers love food with

a lot of fat, oil, salt, and sugar. People often call this kind of food "junk food".

      But bad eating habits go beyond fast food. We find many teenagers eat unhealthily. Some don't have

breakfast before they go to school. During the day, some don't have a proper meal for lunch. In a recent

survey at one school, scientists found that over two-thirds of the students didn't follow a healthy diet. They didn't like vegetables, and many of  them didn't like to eat fruits. They would rather eat food with a lot of

salt, sugar, or fat.

     Parents today also worry about their children's diet. Some doctors give the following advice:

     Teenagers shouldn't eat too much junk food.

     Teenagers shouldn't eat food with too much salt. Salt can cause high blood pressure in the future.

     Teenagers should eat food with less fat, oil, and sugar.

     Teenagers need to eat some fruits and vegetables every day. Fruits and vegetables are rich in vitamins

and have little fat.

     Teenagers need to drink more milk. Milk will help their bones grow.

     Teenagers need to eat breakfast every day. This is good for their body and mind.

1. "Junk food" contains a lot of ____________.

A. milk an sugar                            

B. fruits and sugar

C. vegetables and salt                        

D. fat, oil, salt and sugar

2. In a recent survey at one school, scientists found many students liked _________.

A. vegetables    

B. unhealthy food    

C. fruits      

D. healthy food

3. In this passage, doctors think that teenagers should eat more _____________.

A. food with no vitamins    

B. fat and sugar  

C. vegetables and fruits    

D. salt and oil

4. The passage talks mainly about ______________.

A. bad habits      

B. junk food    

C. unhealthy food    

D. advice on healthy eating

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