It can be inferred from the passage that by 1870 ______.
A.the steam engine had been invented
B.a national commission on agriculture had been established
C.the U.S. horse population was about 10 million
D.technology began to be more economical
第6题
A.滴定速度
B.热源强度
C.煮沸时间
D.样品预测次数
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