Assertion (A): Viruses are considered obligate intracellular parasites. Reason (R): They lack their own metabolic machinery and require a host cell to multiply. MCQ with Answer and Explanation

Explanation:
The ozone layer in the stratosphere protects Earth from harmful ultraviolet (UV) radiation. Its depletion is primarily caused by Chlorofluorocarbons (CFCs), which were widely used in refrigerants, aerosols, and solvents. UV radiation breaks down CFCs, releasing chlorine atoms that catalytically destroy ozone molecules. The Montreal Protocol (1987) was an international treaty to phase out the production of CFCs.

Explanation:
Nitrogen fixation is the process by which atmospheric N₂ is reduced to ammonia, which is then converted into nitrates that plants can absorb. This can be biological (by bacteria like Rhizobium, Azotobacter), physical (lightning), or industrial (Haber-Bosch process). Nitrification is the oxidation of ammonia to nitrates. Ammonification releases ammonia from organic matter. Denitrification converts nitrates back to N₂.

Explanation:
Amoebic dysentery (amoebiasis) is characterized by bloody, mucus-containing stools, caused by Entamoeba histolytica, transmitted through contaminated food/water. Malaria is Plasmodium, sleeping sickness Trypanosoma, leishmaniasis Leishmania.