— May I do the homework tomorrow?

一 No, you______. You should hand in before class tomorrow.

A. mustn't    B. needn't           C. couldn't

有一电池，当两端接3Ω电阻时，输出电流为1.0A，电池内电压为0.6V，该电池的电动势E=      V，内阻r=       Ω，电池的输出功率为       W，此时电源的效率为       。