The above set leads to following conclusions.
(i) Because Gas (B) is colourless and turns acidified K2 Cr2 O7 solution green, it appers to be H2 S.
(ii) H2 S gas is obtained by the reaction of dil. H2 SO4 on A, thus A must be a sulphide.
(iii) Zns sulphide is white thus it indicates that A is ZnS