Two reasons why the Indo-Gangetic plains witnessed the rise of big cities are:

1. Fertile Soil and Water Availability:

The Indo-Gangetic plains are known for their fertile soil and abundant water resources, making them suitable for agriculture. As a result, early settlers in the region were able to cultivate crops and establish permanent settlements. Over time, as these settlements grew and prospered, they evolved into cities. The availability of water resources like rivers also facilitated trade and commerce, leading to the growth of big cities.

2. Trade and Commerce:

The Indo-Gangetic plains have been a hub of trade and commerce since ancient times. The region was strategically located on the trade routes between the east and the west, which made it an important center of trade. The cities that developed in the region served as centers of commerce and were known for their markets, where goods from all over the world were traded. As these cities prospered, they attracted more traders and merchants, leading to further growth.

In summary, the Indo-Gangetic plains witnessed the rise of big cities due to the availability of fertile soil and water resources, which facilitated agriculture and trade, and the strategic location of the region on the trade routes between the east and the west.