A Muon Formed High in the Earth's Atmosphere Travels at a Speed

A muon formed high in the Earth's atmosphere travels at a speed 0.990c for a distance of 6.40 km before it decays. What is the muon's lifetime as measured in its reference frame?

A) 4.65 *10 ^{- 6} s
B) 1.55*10 ^{- 5} s
C) 2.18 *10 ^{- 6} s
D) 3.04 *10 ^{- 6} s