题干

两颗人造地球卫星AB,分别在不同轨道上绕地球做匀速圆周运动,两卫星的轨道半径分别为rArB,且rA> rB,则AB两卫星比较,下列说法正确的是

A:卫星A受到地球引力较小

B:卫星A的运行速率较小

C:卫星B的运动周期较大

D:卫星A的机械能较大

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答案(点此获取答案解析)

B

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    What is the strongest part of your body? Your head? Bones? It's your teeth! There is a special thing called enamel (牙釉质) in teeth. It is the hardest thing in the human body and keeps bacteria (细菌) away. However, teeth are vulnerable (易受伤害的) to problems and disease. September 20 is National Teeth-loving Day in China. Let's pay attention to the health of our teeth.
    Tooth     (1)   are    (1)   around the world. Most children and young adults have tooth decay (龋齿). A study published on China Health Monthly magazine this year showed that 40.74 percent of a sample (抽样) of 1,306 Chinese students under age 15 had decayed teeth.
(2)Food that is rich in sugar increases the risk of tooth decay. After you eat, bacteria go crazy over the sugar on your teeth, like ants at a picnic. The bacteria turn sugar into acids (酸) that eat away at tooth enamel.This cause holes in your teeth.
    Another common problem is crooked teeth (龅牙). When your new teeth begin to grow and the old teeth haven't fallen out, there isn't enough space for the new ones. This causes crooked teeth.
    Don't worry. Here are some ways to keep your teeth healthy.
    Brush at least twice a day – in the morning and before bedtime.
    Learn to use dental floss (牙线). The floss gets rid of food that's hidden where your toothbrush can't get it.
    Visit a dentist. He or she can check if you have any dental problems and help you clean your teeth.
    Pay attention to what you eat. It is better to eat lots of fruits and vegetables instead of fries and drink water instead of soda.
    Wear braces (牙套) to make your teeth straight and beautiful.