The root nodules of leguminous plants contain certain nitrogen fixing bacteria which absorb the atmospheric nitrogen and convert it into nitrogeneous compounds useful for the plants reviving soil fertility. 

The seals found at Indus valley sites gave evidance of overseas trade, which was possible only with the knowledge of art of navigation.
Thus, both A and R are true & R correctly explains A.

Most Himalayan rivers originating in Himalayan peaks are perennial because they are fed by the melting snow throughout the year. 

A vaccine contains the inactivated germs of the disease. But the AIDS virus changes its genetic code and so on vaccine has been invented for it. 

The carbon dioxide envelope in earth's atmosphere traps the heat. With increase in the proportion of carbon dioxide, therefore, the global temperature would rise, thus causing the polar ice to melt 

Most Himalayan rivers are perennial because they are fed by the melting snow throughout the year.
Thus, both A and R are true & R correctly explains A.

Carbon monoxide, when inhaled, combines with haemoglobin of blood to form carboxyhaemoglobin which inhibits the transport of oxygen. 

When in winter, hot water is poured in the glass tumbler. its inner surface tends to expand while the Outer surface in contact with cold atmosphere does not. This opposite interaction causes the tumbler to break. 

Alluvial soils comprising of silt and sand carried down by the rivers, are very fertile. However, cotton is grown in black soil that suits its mineral requirements 

Iron expands in summer. So, gaps are left between rails to allow for expansion.