Transition metal and their compounds are used as catalyst in Industry and in biological system for example in the contact process vanadium compounds in the `+5` state `V_(2)O_(5)` and `VO_(3)^(-)` are used to oxidise `SO_(2)` to `SO_(3)` `SO_(2) + 1/2 O_(2) overset(V_(2)O_(2))rarr SO_(3)` It is thought that the actual oxidation process takes place in two stages. In the first step `V^(+5)` in the presence of oxide ions converts `SO_(2)` to `SO_(3)`. At the same time `V^(+5)` is reduced to `V^(+4)` `2V^(+5) + O^(-2) + SO_(2) rarr 2V^(+4) + SO_(3)` In the second step `V^(+5)` is regenerated from `V^(+4)` by oxygen `2V^(+4) + 1/2 O_(2) rarr 2V^(+5) + SO_(3)` The overall process is of course the sum of these two steps. `SO_(2) + 1/2 O_(2) rarr SO_(3)` Transition metals and their compounds catalyze reactions becauseA. They have completely filled s-subshellB. They have a comparable size due to poor shielding of d-subshellC. They introduce an entirely new reaction mechanism with lower activation energyD. They have variable oxidation sates differ during the course of reaction.