In an oscillator, for sustained oscillations, Barkhausen criterion is `Abeta` equal to (A = voltage gain without feedback and `beta` = feedback factor)
A. increases the input voltage
B. is always in phase with the input voltage
C. is always in antiphase or `180^@` out of phase with the input voltage
D. transfers a part of the output energy of the amplifier to the resonating L-C circuit
