The marble statues are often slowly corroded when kept in the open for a long time. Give a suitable reason.

(N/A) Chemically,marble is $CaCO_3$ (calcium carbonate). When exposed to the atmosphere for a long time,it reacts with acidic pollutants like $SO_2$ and $NO_2$ present in the air,which dissolve in rainwater to form acids (acid rain).
These acids react with the calcium carbonate of the marble to form soluble salts,which are then washed away by rain,leading to the slow corrosion of the statues.
The chemical reaction is as follows:
$CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq)$