Repairing of a machine consists of 5 steps that must be performed sequentially. Time taken to perform each of the 5 steps is found to have an exponential distribution with of mean of 5 minutes and is independent of other steps. If these machines break down in Poisson fashion at an average rate of 2/hour and if there is only one repairman, the average idle time for each machine that has broken down will be

• A. 120 minutes
• B. 110 minutes
• C. 100 minutes
• D. 90 minutes