Compound a (c9h12) shows prominent peaks in its mass spectrum at m/z 120 and 91. compound b (also c9h12) shows prominent peaks at m/z 120 and 105. on vigorous oxidation with chromic acid, both compounds give benzoic acid.

Compound A:   Acetophenone

Compound B:  Propyl Benzene

The mass spectrum for compound A has a base peak at m/z = 105 which is for benzylidyneoxonium ion, and Compound B has a stable peak at m/z = 91 which is for Benzylium cation and rearranges to tropyllium cation and giving a peak at m/z at 91. And both compound A and B when treated with chromic acid produces Benzoic Acid.