She has studied in this school _______ she was seven years old. [ ]

A. since

B. if

C. until

D. after

一个电阻负载两端电压为U，所消耗的功率为P。流过负载的电流I和负载的电阻R分别为：（U、I、R、P的单位分别为伏特、安培、欧姆、瓦特，”x＾m”表示“x的m次方”）（）

A.I=P/U＾2，R=P×U

B.I=P/U，R=U＾2/P

C.I=P×U，R=P/U

D.I=U/P，R=P/U