Which of the following is/are believed to be the causes for the collapse of the Indus Valley Civilization?

1. Immigration
2. Deforestation
3. Climate change
4. Decline in trade

Around 1800 BCE, signs of a gradual decline began to emerge, and by around 1700 BCE, most of the cities were abandoned. In 1953, Sir Mortimer Wheeler proposed that the decline of the Indus Civilization was caused by the invasion of Aryans. Today, many scholars believe that the collapse of the Indus Civilization was caused by drought and a decline in trade with Egypt and Mesopotamia. It has also been suggested that immigration by new peoples, deforestation, floods, or changes in the course of the river may have contributed to the collapse of the IVC. A possible natural reason for the IVC's decline would have been connected with climate change that is also signalled for the neighbouring areas of the Middle East. A tectonic event may have diverted the system's sources toward the Ganges Plain, though there is complete uncertainty about the date of this event, as most settlements inside Ghaggar-Hakra river beds have not yet been dated. The actual reason for decline might be any combination of these factors.