Trade developed early and was very important to the people of sumer because they?

Trade was important to the Sumer because there certain resources that the Sumerians didn’t have.  These were stone, wood and metals like copper.  Due to this lack of resources, the Sumerians establish trade with other tribes as well as merchants.  One is when inhabitants of the Zagros and Taurus mountains bartered traded timber, limestone, gold, silver and copper with the Sumerians.  The Sumerians gave them wheat and barley in exchange.  Apart this, boats were developed enabling them to transport goods for trade.

Here is a picture  of the net. its 4 squares in a row diagonal and on the third draw one square aove and under

I'm pretty sure it's c. extremely gradual but not 100% sure though.