4. An ideal gas is allowed to undergo a free expansion. If its initial volume is V1 and its finalvolume isV2 , the change inentropy is
(A) nR ln ( V2 /V1 )
(B) nRT ln ( V_{2 /V1 ) 
(C) nk ln ( V2 /V1 ) 
(D) 0
(E) nR}