Q. Help me settle a bet: Which U.S. airline carries the most passengers? I’m saying Southwest, but my wagering opponent says American. Which is it?

A. You both lose. If you had made that bet a year ago, you would have won. But since the merger of Northwest into Delta finally became official and complete last year, Delta now carries more flyers than any other U.S. airline.

According to the latest numbers form the Bureau of Transportation Statistics, Delta carried 33.6 million passengers on domestic and international routes during January-April 2010. That compares with 32.6 million for number two Southwest and 27.3 million for number three American. (For some reason, BTS lists American Eagle separately, but even if you add in the 5.0 million January-April passengers on Eagle, the combined American total is still slightly below Southwest.) A year ago, Southwest ranked number one, followed by American and third-place Delta. In fourth place this year – as it was last year – is United, with a four-month total of 16.9 million passengers, followed by US Airways (16.3 million), Continental (13.7 million), JetBlue (7.53 million), SkyWest (7.50 million), and AirTran (7.49 million).