ΔU=nCv​ΔTFor isothermal process T is constantSo ΔU=0A→IIAdiabatic processΔQ=0ΔQ=ΔU+ΔWΔU=−ΔWWork done by gas is positiveSo ΔU is negativeB→IFor Isochoric process ΔW=0C→IVFor Isobaric processΔW=PΔVî€ =0ΔU=nCV​ΔTî€ =0Heat absorbed goes partly to increase internal energy and partly do work.