HELP! I NEED HELP TO UNDERSTAND THIS! In this graph below, Label A represents the gases hydrogen sulfide, methane, and carbon dioxide. What gases are represented by label B and how do these gas levels relate to the evolution of photosynthetic organisms?

The independent variable is the type of music.the dependent variable is the pulse rate.the intervening variable is the age of the subject.if the mother does not like rock, her pulse is likely to not increase.the same might be true for the sister.the friend may like rap and gets excited.to establish cause/effect relationship, all of the subjects must be consistent.hope i .