Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: The plains of Ganga and the Indus are formed by basins of three distinct river systems e.g. Indus, Ganga and Brahmaputra.
Reason R: These plains are one of the least populated areas in India.
In the light of the above statements, choose the most appropriate answer from the options given below