What was the significance of aryan and other tribal migrations on the indus valley?

      The Indo-Aryan migration theory says that Indo-Aryan languages were introduced into Anadolia, the northern part of the Indian subcontinent and inner Asia during the 2nd millenium BC.
      Indus Valley Civilization was a Bronze Age civilization ( 3300 - 1300 BC ) in the northwest regions of South Asia ( today: northeast Afghanistan, Pakistan and northwest India ). It flourished in the basins of Indus river. It is also known as the Harrapan civilization. Around 1800 BC signs of decline of that civilization began to emerge. Many scholars believe that a decline in trade with Egypt and Mesopotamia and the climate change caused the collapse of the Indus Valley Civilization ( not an invasion of the Indo-Aryan tribes ).
    Significance of Indo-Aryan tribal migrations are:
   - spread of the Indo-Aryan languages,
   - spread of the new culture and the Vedic-Brahmanic culture,
   - the new burial rites: cremation burial, dominant in Hinduism today,
   - many new settlements in the Indus Valley began to appear and other increased in size.   

A,) cause a & b are equal & both gives 180°