The development of the Qijiang River will have six characteristics, one of which is to build a leisure and summer resort in the main city and the largest negative oxygen ion bank in the suburbs of the main city. According to the meaning of the question, "oxygen molecules in the air can form negative oxygen ions when they get electrons", and when they form ions, the electrons change and the protons remain unchanged; So the difference between O2 2-and O2 is the number of electrons; The number of oxygen atoms is 2; Because the number of protons and neutrons remains the same, the relative atomic mass remains the same.

So choose B.