The explanatory variables (is) are what you manipulated, the response variables is (are) what you are interested in, and what you are measuring. You always want to know how your independent variables influence the dependent variable(s).
If you have a look at this graph, which of the following is true?
1) Biomass is your independent variable
2) Temperature and CO2 are your explanatory variables
3) Biomass is your explanatory variable
4) This experiment had three treatments