Why is the exchange of rings linked to marriage in so much of the world and are there any widespread alternatives?

Wedding rings have been exchanged in marriage ceremonies since the Ancient Egyptians.  The Egyptians believed that the circle was the symbol of eternity.  The Romans continued the tradition and believed that the ring symbolized fidelity and was associated with the dowry.  An alternative in the United States that is becoming popular is having a tattoo on your left ring finger.  This is not a widespread custom around the world. 

Ithink the answer is b.)