在“测定小灯泡的额定功率”的实验中，某同学电路连接正确，闭合开关，灯泡发光，但测试中无论怎样调节滑动变阻器，电压表示数都达不到灯泡的额定电压值，其原因可能是[ ]

A、变阻器总电阻太大

B、电压表量程太大了

C、电源电压太低

D、灯泡灯丝断开

My father served in the army in _____ when he was in _____. [ ]

A. 1950's; twenties

B. the 1950's; his twenties

C. the 1950's; the twenties

D. 1950's; the twenties