Which are thought to have contributed to the decline the way of the idus valley civilization?

There are numerous theories surrounding the decline of the Indus valley Civilization. Some of the experts are of the opinion that there might have been mass invasion that resulted in the decline. Some other experts feel that natural disaster could have brought the civilization to a premature end. There are also theories that the climate of the place might have suddenly become dry resulting in the destruction of the civilization. experts have also not ruled out the possibility of an epidemic that might have resulted in the collapse of the Indus Valley Civilization.

Ithink the answer is d i am not really sure tho