Two thermally insulated vessels 1 and 2 are filled with air at temperatures (T1 , T2), volume (V1 , V2) and pressure (P1 , P2) respectively. If the valve joining the two vessels is opened, the temperature inside the vessel at equilibrium will be
(a) T1 + T2
(b) (T1 + T2)/2
(c) (T1T2(P1V1 + P2V2))/(P1V1T2 + P2V2T1)
(d) (T1T2(P1V1 + P2V2))/(P1V1T1 +P2V2T2).