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