In this particular case, the carbon contains a single oxygen atom. But in this particular case, the carbon contains one two oxygen atoms. So somewhere in this reaction, there was a loss of an oxygen atom. Likewise, if we take a look at the nitrogen and compare it to this nitrogen, in this particular case, we have three H atoms. In this particular case, we only have one. So when we basically combine A and B to form C, we basically lose two H atoms and one oxygen, and that is a water molecule.