Where do most carbon monoxide emissions come from? cars and trucks factories planes and trains boats

 D.  Boats, Because, your not able to smell the carbon monoxide on boats, you can't smell it on planes, and you CAN smell it on trains so that rules 'C.' out.  Your able to smell it on land vehicles, don't believe me? Stick your nose in the back of a car that's running.  Your able to smell it in factories, because in China, individuals have to wear gas masks to go outside or walk to work.

the answer to your question is b