The average transaction at a single channel, single phase checkout stand can be completed in $5$ minutes and customers arrive at the average rate of one every $4$ minutes. On averag what is the server utilization? $($Choose the closest answer.$)$

a$.0\mathrm{.}25$

b$.1\mathrm{.}25$

c$.0\mathrm{.}80$

d$.0\mathrm{.}63$