c
${(\Delta Q)_V} = \mu \,{C_V}\Delta T = {\rm{W}}\,\Delta T$ $(W =$ heat capacity$)$
$\Rightarrow$ $W = \frac{{{{(\Delta Q)}_V}}}{{\Delta T}} = \frac{{\Delta U}}{{\Delta T}}$ $[\because \,\,{(\Delta Q)_V} = \Delta V]$
$\Rightarrow$ $W = \frac{{80}}{{(120 - 100)}} = 4\,J/K$