A particle of mass m moving with speed v hits elastically another stationary particle of mass 2m inside a smooth horizontal circular tube of mean radius r. The time after which the second collision will take place is

• A

  $\frac{2\pi r}{v}$

• B

  $\frac{4\pi r}{v}$

• C

  $\frac{3\pi r}{2v}$

• D

  $\frac{\pi r}{v}$