[单项选择]Successful innovations have driven many older technologies to extinction and have resulted in higher productivity, greater consumption of energy, increased demand for raw materials, accelerated flow of materials through the economy and increased quantities of metals and other substances in use per person. The history of industrial development is full of examples.
In 1870, homes and mules were the prime source of power on U.S. farms. One horse or many decades. At that time, had a national commission been asked to forecast the horse and mule population for 1970, its answer probably would have depended on whether its consultants were of an economic mm of mind. Had they been "economists", they would have recognized that the power of steam had already been harnessed to industry and to land and ocean transport. They would have recognized further that would be only a matter of time before steam would be the prime source of power on the farm. It would have been difficult for them to avoid
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