What caused the decline in the Zoroastrian population in Persia? A: An epidemic broke out in Persia, which resulted in large-scale deaths among Zoroastrians. B: The invasion of Persia by Islamic rulers forced the local Zoroastrians to convert to Islam. C: A natural disaster struck Persia and caused a decline in Zoroastrian numbers. D: King Vishtaspa implemented humiliating laws on local Zoroastrians, forcing them to convert to Islam to avoid ill treatment.