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%