In $\odot(O, 4)$,$\widehat{ACB}$ is a minor arc and $m \angle AOB = 45^\circ$. Then,the length of minor $\widehat{ACB}$ is $\ldots \ldots \ldots \ldots$

• A
  $\pi$
• B
  $2\pi$
• C
  $3\pi$
• D
  $4\pi$