Suppose that the running time for each process in milliseconds is an exponential random variable with parameter λ=1/20. If process P1 arrives immediately ahead of the process P2 in the running state, then the probability that process P2 will have to wait more than 20 milliseconds is _____________ .
A

0.274 
B

0.324 
C

0.428 
D

0.368 
How to approach this. even not able to understand the question.