Consider a single server queuing model with Poisson arrivals (l=4/hour) and exponential service (m =4/hour). The number in the system is restricted to a maximum of 10. The probability that a person who comes in leaves without joining the queue is
(a) $\frac{1}{11}$ 
(b) $\frac{1}{10}$ 
(c) $\frac{1}{9}$ 
(d) $\frac{1}{2}$ 