单选题
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
【正确答案】
A
【答案解析】
[注释] 根据第二段开始部分，1870年时马和骡子为美国农场的主要劳动力，尽管那时已出现蒸汽机。其他选项的信息有误或没有在短文中涉及。
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