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