单选题 The (three-fields) system is an agricultural method in which one field (out of) (three) is (left) to lie fallow while the other two fields are being cultivated.
【正确答案】 A
【答案解析】单复数错误，应改为three-field。“数词+名词：”构成的合成形容词中，名词要用单数。如：an 80-year-old man。