as.factor()函数可以将字符串变量改变为因子变量
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【多选题】询问天气的表达方式有：
A. How is the weather?
B. What is the weather like?
C. What is the number?
【多选题】描述天气的句子有：
A. It's so stuffy and no wind at all.
B. It's warm and sunny.
C. Very hot and dry.
D. The heat is killing me.
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