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答案：A
解析：
In the question, you are told that 7 gallons of gasoline can plow 3 acres and you are asked to find how many acres, call it 
, can be plowed by 
gallons. You can set up a ratio of the number of gallons of gasoline used to the number of acres plowed. Therefore, you have 
. Cross-multiplying and solving for 
gives you the answer of 
acres, or 
acres.
