Pi Mesons at Rest Have a Half-Life of T $\beta$ C, the Distance in Which the Intensity of the Beam

Pi mesons at rest have a half-life of T.If a beam of pi mesons is traveling at a speed of v = $\beta$ c, the distance in which the intensity of the beam is halved is:

A) "c $\beta$ T(1 - $\beta$ ²) ^-1/2"
B) "c $\beta$ T[(1 + $\beta$ ) /(1 - $\beta$ ) ]^1/2"
C) " $\beta$ vT"
D) "(1 - $\beta$ ²) ^1/2vT"
E) "none of the above"