Option A is a reasonable possibility, since the some students could take the center of the "curve" as a reference, and some other the top or the bottom of it. Option A applies.
If the wrong liquid is chosen, it would not cause changes since students are going to measuere volumes. One liter of milk is the same volume of one liter of water. (The same volume, but not the same mass). Then, B does not apply.
If some students try to be more precise than others, it would definitely change de results between students. C applies.
If we found that A and C apply, then, D does not apply since we have stablished that there are factors that can cause difference in measurements.