Answer: d. they disappeared as though into thin air, and the cause remains unknown.
The ancient peoples of the Indus Valley, also known as Harappans, disappeared during the later half of the 2nd millennium BCE. This was around a thousand years before the beginning of the Gupta period.
The cause of their disappearance is a matter of debate. Bodies from around that time show more signs of violence and disease. Some theories that have been proposed are an Aryan invasion, climate change, droughts and earthquakes. However, the cause is still unknown.
