( )对象代表性的具体标准，根据每次市场调查的目的和调查对象的特点来确定。
A．市场普查B．重点调查C．典型调查D．抽样调查
参考答案：C
如图所示电路，电源电压U=10V，定值电阻R0=20Ω，当开关K1闭合、K2断开后，调节滑动变阻器使小灯泡正常发光，此时电流表读数I1=0.25A；保持滑动变阻器滑动触头的位置不变，再断开K1、闭合K2，电流表读数I2=0.20A．求小灯泡的额定功率．
I became a teacher ten years ______. I have been a teacher _______ 2008.
A．ago; for
B．before; since
C．ago; since
D．ago; in
