[" 30.A particle of mass "m" moving with speed "v" hits "],[" elastically another stationary particle of mass "2m],[" inside a smooth horizontal circular tube of radius "r" ."],[" The time after which the second collision will take "],[" place is "],[[" (a) "(2 pi r)/(v)," (b) "(4 pi r)/(v)," (c) "(3 pi r)/(2v)," (d) "(pi r)/(v)]]