在探究光对鼠妇生活的影响的实验中，要控制的变量是（    ）

    We know music is very important. Do you notice music playing at any of those places when you go somewhere? Today most stores, stations, restaurants and other places play music. You might even hear music in an office or on a farm.

    Scientists believe that music influences the way people behave. They think that the sound of western classical music makes people feel richer. When a restaurant plays classical music, people spend more money on food and drinks. When the restaurant plays modern music, people spend less money. Without music, people spend even less.

    Scientists also believe that loud, fast music makes people eat faster. Some restaurants play fast music during their hours. This makes people eat faster and leave quickly. Restaurants can make more money in this way.

    Some scientists think that music makes you think and learn better. They say that music helps students to be more active. It is true that people learn better when they are relaxed. And listening to music can help you relax.

    The next time you hear music somewhere, be careful. It might influence the way you do things.

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