BOTH INTERNATIONAL & INLAND While underway in fog you hear a vessel sound one prolonged blast followed by two short blasts. What does this signal indicate?

Under Inland Navigation Rules, for the condition where 'While underway in fog you hear a vessel sound one prolonged blast followed by two short blasts. What does this signal indicate', the correct requirement is: 'A vessel towing'. This standard ensures clear navigational intent and collision avoidance.