While on a VFR cross country and not in contact with ATC, what frequency would you use in the event of an emergency?

Explanation

121.5 MHz is the international VHF emergency frequency. It is monitored by ATC facilities, many FSSs and military facilities, so a pilot who is not already talking to ATC should use it to declare a distress or urgency situation (AIM 6-3-1). 122.5 MHz is a flight service frequency and 128.725 MHz is not an emergency frequency, so neither is guaranteed to reach an emergency service.