In 530 BC Cyrus the Great, King of the Persian Achaemenid Empire crossed the Hindu-Kush mountains to seek tribute from the tribes of Kamboja, Gandhara and the trans-India region (modern Afghanistan and Pakistan). By 520 BC, during the reign of Darius I of Persia, much of the northwestern subcontinent (present-day eastern Afghanistan and Pakistan) came under the rule of the Persian Achaemenid Empire, as part of the far easternmost territories. The area remained under Persian control for two centuries. During this time India supplied mercenaries to the Persian army then fighting in Greece.
What king ruled Persia in 520 BC? (If the question is unanswerable, say "unanswerable")
Darius I of Persia