It can be inferred from the passage that by 170______.
A.technology began to be more economical
B.the steam engine had been invented
C.the U. S. horse population was about 10 million
D.a national commission on agriculture had been established

A.A.technology
B.the
C.the
D.a

【参考答案】

B
解析：推断题。第二段第五句从technologists预测的分析中得出，他们已经认识到蒸汽的力量，所以蒸汽机......

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