Assuming a $24$ hour operation in a warehouse, and an arrival following Poison distribution with mean rate of $36$ per day and service $($unloading using a single bay$)$ following exponential with a rate of $48$ trucks per day, answer the following question. What is the expected number of trucks in the system?$2\mathrm{.}25$B $11\mathrm{.}5$D $3$E $2$