题目

 Leo _______ to China in 2008 so he ______________ in China for about 4 years. A. came; has been teaching           B. has been; has been teaching C. came; have been teaching          D. has gone; have been teaching 答案：A蒸汽机从发明到应用经历了84年，电动机为65年，原子能的利用为6年，晶体管为4年，这突出说明了（ ）A．科学技术是第一生产力B．从科学到技术再到生产的周期大大缩短C．科技发展速度快D．新技术更新换代的速度加快