In addition to tributary relations with the Kushans, the Han Empire received gifts from the Parthian Empire, from a king in modern Burma, from a ruler in Japan, and initiated an unsuccessful mission to Daqin (Rome) in AD 97 with Gan Ying as emissary. A Roman embassy of Emperor Marcus Aurelius (r. 161180 AD) is recorded in the Hou Hanshu to have reached the court of Emperor Huan of Han (r. AD 146168) in AD 166, yet Rafe de Crespigny asserts that this was most likely a group of Roman merchants. Other travelers to Eastern-Han China included Buddhist monks who translated works into Chinese, such as An Shigao of Parthia, and Lokaksema from Kushan-era Gandhara, India.
Try to answer this question if possible (otherwise reply "unanswerable"): When did Emperor Huan's reign begin?
AD 146