$A$ stone tied to the end of a string $1\,m$ long is whirled in a horizontal circle with a constant speed. If the stone makes $22$ revolutions in $44$ seconds,what is the magnitude and direction of the acceleration of the stone?

• A
  $\frac{\pi^2}{4}\,m/s^2$ and direction along the radius towards the centre
• B
  $\pi^2\,m/s^2$ and direction along the radius away from the centre
• C
  $\pi^2\,m/s^2$ and direction along the radius towards the centre
• D
  $\pi^2\,m/s^2$ and direction along the tangent to the circle