Jobs arrive at a facility at an average rate of 5 in an 8 hour shift. The arrival of jobs follows Poisson distribution. The average service time of a job on the facility is 40 minutes. The service time follows exponential distribution. Idle time (in hours) at the facility per shift will be
(a) $\frac{5}{7}$ 
(b) $\frac{14}{3}$ 
(c) $\frac{7}{5}$ 
(d) $\frac{10}{3}$ 