When `KMnO_(4)` solution is added to hot oxalic acid solution, the decolourisation is slow in the beginning but becomes instantaneous after some time. This is because.
A. `Mn^(2+)` acts as auto catalyst
B. `CO_(2)` is formed
C. Reaction is exothermic
D. `MnO_(4)^(-)` catalyses the reaction