oxygen reacting with the mineral in roco
The answer is: the amount of the saliva samples must stay consistent while the amount of starch must vary.
The amount of the saliva samples must stay consistent because amylase, present in the saliva, is the independent variable which affects the dependent variable - the amount of starch. So, amylase will do its job while remaining surface area of the starch will depend on the starting amount of the starch.
Your answer would be B. The first one would be correct if the question was “how do we identify both bloodstains are from the victim”.