In a good experimental design, the part of the experiment that is intentionally changed is the A dependent variable.
B independent variable.
C constant
D control

Constants never change.

Controls typically don't change or they are done by standard means

(like in an experiment where you water plants with sodas, the plant you use water with is the control)

Dependent variables are the outcome of the experiment. This is the result of changing something.

Independent variables are what you are testing. A independent variable is something that is  intentionally altered.

easy way to remember it is that dependent variables depend on what other variables are. so you cant change a dependent variable, but you can change the other variables.

photosynthesis makes the glucose that is used in cellular respiration to make atp. the glucose is then turned back into carbon dioxide, which is used in photosynthesis. while water is broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water.