---I’m a little discouraged, for I didn’t do the job well

---Come on , boy. Everybody is    to make a mistake.

A．possible

B．probably

C．likely

D．possibility

两只内阻不知的电压表串联后接在电源的两极间，表A的读数8V，表B读数5V，表B单独接在电源两极间，表B的读数为10V，求电源电动势