题干

下面是一些正在振动的物体:甲:手臂以每秒1次的频率上下挥动;乙:蝙蝠的嘴发出频率为105Hz的振动;丙:用小锤敲音叉,音叉发生256Hz的振动,以下说法正确的是(  )

A:我们只能听见音叉发出的声音,所以丙是声源,其他不是声源

B:甲、乙、丙都是声源,我们都能听到它们发出的声音

C:甲、乙、丙都是声源,我们只能听到丙发出的声音

D:甲和丙是声源,我们能看见振动或听到声音;乙不是声源,因为我们既看不见振动,也听不到声音

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答案(点此获取答案解析)

C

同类题4

    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.