$6-159$ Consider a Carnot refrigerator and a Carnot heat pump operating between the same two thermal energy reservoirs. If the COP of the refrigerator is $3\mathrm{.}4,$ the COP of the heat pump is

$($a$)1\mathrm{.}7$

$($b$)2\mathrm{.}4$

$($c$)3\mathrm{.}4$

$($d$)4\mathrm{.}4$

$($e$)5\mathrm{.}0$