By about 1700 B.C.E., the residents of Harappa and Mohenjo-daro began to abandon their cities because
A) frequent epidemics made city living impossible.
B) deforestation of the Indus River valley brought about ecological degradation.
C) the horse-riding Aryans began to invade the cities.
D) the Indus River dried up.
E) All these answers are correct.