Explanation:

Both A and R are true but R is not a correct explanation of A.

With an area of around 3.75 lakh sq.km, this is the largest section of the Northern Plains. This region includes sediments brought down and deposited by the Himalayan rivers as well as the Peninsular rivers. Himalayan rivers include the Ganga and its tributaries such as Yamuna, Gandak, Kali, Kosi etc. and Peninsular rivers include such as Chambal, Ken, Betwa, Son etc. Since the plain is gently sloping towards the southeast, most of the rivers that flow through this region drain into the Bay of Bengal. The Ganges, or Ganga, is a trans-boundary river of Asia which flows through India and Bangladesh. The 2,601 km river rises in the western Himalayas in the Indian state of Uttarakhand, and flows south and east through the Gangetic Plain of India and Bangladesh, eventually emptying into the Bay of Bengal.