The acids present in the acid rain react with the marble of monument and corrode it slowly.
The slow corrosion (or eating up) of marble of a monument by acid rain is known as Marble cancer.
The main cause of marble cancer is acid rain.
Various gases like sulphur dioxide, oxides of nitrogen are released from factories.
These gases react with the water vapour present in the atmosphere and form acids like sulphuric acid, nitric acid etc.
These acids mix with rain and cause acid rain.
This corrodes the marble of the monuments.