given variables list them as independent (x) or as a dependent (y)
I weight of child; prescription dosage
II monthly commision; monthly sales
II ability to concerntrate; level of fatique
IV crop yield; rainfall
An independent variable is the potential stimulus or cause, usually directly manipulated by the experimenter, so it could also be called a manipulative variable.
A dependent variable is the response or measure of results.
Try to figure which factor will effect the other. That will be the independent variable.