An alternating e.m.f. of frequency $\mathrm{v}=\mathrm{\hspace{0.17em}}\left(\frac{1}{2\mathrm{\pi }\sqrt{\mathrm{LC}}}\right)$ is applied to a series LCR circuit. For this frequency of the applied e.m.f. 

• A

  The circuit is at resonance and its impedance is made up only of a reactive part

• B

  The current in the circuit is in phase with the applied e.m.f. and the voltage across R equals this applied emf

• C

  The sum of the p.d.’s across the inductance and capacitance equals the applied e.m.f. which is ${180}^{\mathrm{o}}$ ahead of phase of the current in the circuit

• D

  The quality factor of the circuit is  $\mathrm{\omega L}/\mathrm{R}$ or $1/\mathrm{\omega CR}$  and this is a measure of the voltage magnification (produced by the circuit at resonance) as well as the sharpness of resonance of the circuit