Which factor should be kept the same in a fair test of the effect of soap amount?

Correct answer: Volume of water

Direct answer: Option C is correct. The investigation tests the effect of soap amount, so soap amount is the independent variable and should be changed deliberately. Foam height is the measured dependent result. To make the comparison fair, the volume of water should remain the same in every trial. Reasoning: use equal water; change only the soap amount; measure the foam height; compare the results. Option A is wrong as the tested factor is supposed to vary, not remain constant. Option B is wrong because foam height is the result being measured. Option C works because equal water removes water volume as an alternative explanation, making the soap effect clearer. Option D is wrong because “measured result” is not a condition kept constant; it is the outcome recorded after each trial. Other relevant conditions, such as container and mixing method, may also need control. Memory cue: keep everything same except the factor being tested.