The time between accidents on the riverfront bridge follows a poisson process with a mean time of 40 days between accidents. the time between accidents on the overview bridge follows a poisson process with a mean time of 20 days between accidents. these two bridges are in separate states. let z- the average of the time until the next accident on the riverfront bridge and the time until the next accident on the overview bridge. the moment generating function of z is then given by:

a. [(1-10t)(1-20t)]^-1
b. [(1-20t)(1-40t)]^-1
c. exp[60(e^t-1)]
d. exp[60e^05t-1)]