Most cities and towns in China have night markets(市场). During the day, these streets are quiet. At night, they become crowded(拥挤的)and lively. Usually, people get there at three or four in the afternoon. By 6:00, the streets are like rivers of people. Night markets are fun places to shop. You can buy clothes, shoes, and many other things. If you think the price is too high, you can bargain with the seller. You can also buy nice food. When you are hungry, you can sit down for a meal. Smelly tofu is popular, but it really smells bad! The best thing is that everything in night markets is cheap!

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小华在“探究滑动摩擦力的大小与什么因素有关”时，设计了如图所示的实验，下面关于实验的说法正确的是（　　）