BOTH INTERNATIONAL & INLAND While underway in fog, you hear a vessel sound one prolonged blast followed by two short blasts on the whistle. 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 on the whistle. What does this signal indicate', the correct requirement is: 'A vessel towing'. This standard ensures clear navigational intent and collision avoidance.