Suppose One Observes Cosmic Rays Creating a New Particle High $\mu$

Multiple Choice

Suppose one observes cosmic rays creating a new particle high in the atmosphere and the particle moves at 99.7% the speed of light. It is observed to decay in an average 37.0 $\mu$ s. What is the proper lifetime of the particle?

A) 2.86 $\mu$ s
B) 18.1 $\mu$ s
C) 5.33 ms
D) 12.4 $\mu$ s
E) 4.33 $\mu$ s