What is power factor and voltage-current relationship in an $AC$ circuit?

• A
  Power factor is $\cos \phi$,where $\phi$ is the phase difference between voltage and current.
• B
  Power factor is $\sin \phi$,where $\phi$ is the phase difference between voltage and current.
• C
  Power factor is $\tan \phi$,where $\phi$ is the phase difference between voltage and current.
• D
  Power factor is $\cot \phi$,where $\phi$ is the phase difference between voltage and current.