(a) Because of release of oxides of nitrogen and sulphur which further cause acid rains and these acid rains have a corroding effect on marble.
(b) Lichens are sensitive to SO2. In Delhi due to more pollution and due to large number of factories lichens do not grow while Manali or Darjeeling has green and humid place which is adequate for the growth of lichens.
(c) Smoke containing fog particles results in the smog.
Harmful effects of smog:
(i) It reduces visibility thus interupting road and air traffic.
(ii) It causes breathing problems.