The correct answer is C) The soil became too salty for crops to grow.
One theory about what caused the decline of the Indus Valley civilization is "The soil became too salty for crops to grow."
Another important cause, according to scientists and archeologists, could have been a great flood that wiped the entire civilization. Let's have in mind that the Indus Valley Civilization settled in next to the Indus River. Other ancient civilizations also settled in next to important rivers because of the many benefits it generated to them That was the case of the ancient Egypt civilization that settled in next to the Nile River banks and the Sumerian civilization that settled in between the Tigris and Euphrates Rivers.
