-- Haven’t I told you to prepare your lessons well before the exam? You failed again.

-- But I ______ for the exam longer than I usually do.  [ ]

A. was preparing

B. will prepare    

C. have prepared  

D. prepared

如图所示，直流电源的电动势E＝30V，内阻r＝1Ω，将它与一个标有“6V 12W”的小灯泡和一台内部导线电阻R₀=2Ω的直流电动机串联组成闭合电路，小灯泡恰能正常发光，不计电动机转动时的摩擦。求：电动机输出的机械功率。