下列运算正确的是（　　）

如图所示，滑板车在沿水平方向运动时，如果人不再用脚蹬地，它最终就会停下来，以下说法正确的是（　　）

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假定英语课上老师要求同桌之间交换修改作文，请你修改你同桌写的以下作文。文中共有10处语言错误，每句中最多有两处。错误涉及一个单词的增加、删除或修改。

增加：在缺词处加一个漏字符号(∧)，并在其下面写出该加的词。

删除：把多余的词用斜线(\)划掉。

修改：在错的词下划一横线，并在该词下面写出修改后的词。

注意：1）每处错误及其修改仅限一词；

2）只允许修改10处，多者（从第11处起）不计分。

    Though English is a hard subject, but I like my English teacher a lot, and so are my classmates. She is nice to us and always tells us stories of the days when she was in her twenty. Sometimes she even stayed in school to help students make up for their lessons. She always encourages us and tells us do our best. We are gratefully to her, so one day we bought him a cake and a gift. We also made a big card for her, on it we wrote all our names. Seeing these, she was moving and had a big smile in her face.

亨利·福特曾说过:“如果我最初问消费者他们想要什么,他们会告诉我‘要一匹更快的马!”。有人说,如果真这样,汽车大王就不会出现了。材料体现的经济学道理是（  ）。