A Particle Physicist Observes Cosmic Rays Creating a New Particle $99.7 \%$

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 $37.0 \mu \mathrm { s }$ . If this particle were at rest in the laboratory, what would be its average lifetime?

A) $4.3 \mu \mathrm { s }$
B) $12 \mu \mathrm { s }$
C) $5.3 \mu \mathrm { s }$
D) $2.9 \mu \mathrm { s }$
E) $18 \mu \mathrm { s }$