单项选择题
A．at this rate B．above all C．on an average D．by way of
A. genes that can kill cancer B. genes that will lead to leukemia C. viruses that can kill AIDS D. genes that will lead to cancer
A．Europe was changing more quickly than America. B．Europe had greater need of farm machinery than America did. C．America was finally running out of good farmland. D．There was a shortage of workers on American farms.