What were the reasons for the decline of the Harappa civilisation?

The correct options are
A Floods at regular intervals in Indus and its tributaries
B Attack from the Aryans
C Overuse of wood for burning bricks that destroyed the forests, which necessitated in their migration
D The spread of infectious diseases
There are many causes for the decline of Harappan civilisation. There were regular floods in the Indus and its tributaries. The overuse of wood for burning bricks that destroyed the forests, which made them migrate. The Aryans, who came from the north, could have attacked them. The spread of infectious diseases could have made an impact as well.