There is a method where you can reduce background noise by selecting a sample of the (constant) background noise (without someone speaking over it) and applying some sort of filter to the track.

1.) I don't know how to do that. I just know that you can.
2.) I don't know if Audacity can do that.
3.) I have used Audacity for a short time in class
4.) You would have to pay me to do that, which would eliminate the purpose of using Audacity.