In a Series RLC Circuit the Rms Value of the Generator

In a series RLC circuit the rms value of the generator emf is and the rms value of the current is i.The current lags the emf by $\phi$ .The average power supplied by the generator is given by:

A) (i/2) cos $\phi$
B) i
C) i²/Z
D) i²Z
E) i²R