Which of the following functions is periodic?

• A

  $f\left(x\right)=x-\left[x\right],\text{ where }\left[x\right]\text{ denotes the largest integer less than or equal to the real numb er x }$

• B

  $f\left(x\right)=\sin \frac{1}{x}\text{ for }x\ne 0,f\left(0\right)=0$

• C

  $f\left(x\right)=x\cos x$

• D

   None of these