According to ICAO, a distress message on 121.5 Mhz should begin

According to ICAO, a distress message on 121.5 Mhz should begin with _______.

A.MAYDAY MAYDAY MAYDAY

B.PANPAN PANPAN PANPAN

C.HELP HELP HELP

D.SOS SOS SOS

正确答案是A