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

Multiple Choice

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