A Particle Physicist Observes Cosmic Rays Creating a New Particle

Multiple Choice

A particle physicist observes cosmic rays creating a new particle high in the atmosphere,and the speed of this particle is measured at 99.7% the speed of light.It is unstable,and is observed to decay in an average 37.0 μs.If this particle were at rest in the laboratory,what would be its average lifetime?

A) 2.9 µs
B) 18 µs
C) 5.3 µs
D) 12 µs
E) 4.3 µs