The gas A is used

a) i) The gas A used by animals to obtain energy from food by respiration is Oxygen O₂.

ii) The gas B is Ozone O₃. Radiations X (ultraviolet) when act on oxygen, convert oxygen to ozone.

b) The layer C is Ozone layer. Gas B i.e. Ozone forms ozone layer in the stratosphere.

c) i) X- Ultraviolet radiations

ii) Y- Sun. Ultraviolet radiations come from the sun

iii) Z- Chlorofluorocarbons. CFC’s are released from refrigerator and air-conditions which are destroying the ozone layer.

d) Ozone hole has been formed over Antarctica’s upper atmosphere.

e) The radiations X i.e. ultraviolet radiations cause skin cancer, when they fall on skin; they destroy the genetic material of the cells, thus resulting in uncontrolled growth of cells. These radiations are also absorbed by the lens of the eyes which leads to formation of free radicals inside the lens that causes cataract over time. Ultraviolet radiations also cause immunosuppression.