Nitrogen is expanded isentropically$.$ Its temperature changes from $620$ F to $60$ F$.$ If the volumetric ratio is V$2/$V$1=6\mathrm{.}22,$ what is the work done by the gas? a$.-120$ BTU$/$lb c$.-540$ BTU$/$lb b$.-111\mathrm{.}3$ BTU$/$lb d$.-99$ BTU$/$lb