下列科技成就中，属于东汉张衡发明的是 [ ]
A．指南针
B．印刷术
C．地动仪
D．火药
答案：C
An excellent teacher is believed to be one who devotes himself to the children but ask for nothing _____. [ ]
A. in turn
B. in advance
C. in short
D. in return
当电网中并入的风力电量达到一定程度，会引起电压不稳定，一般建议不大于（）。
A、5%
B、10％
C、15%
D、25%
