If the effect of water temperature on dissolving rate is tested which factor should be kept constant?

Correct answer: Amount of salt

The direct answer is C: the amount of salt. In a fair experiment, the independent variable is deliberately changed, the dependent variable is measured, and controlled variables are kept the same. Here the question asks how water temperature affects dissolving rate. Therefore temperature is the independent variable, dissolving rate or time is the measured outcome, and the amount of salt should remain constant. Step by step: change the water temperature; use the same quantity of salt each time; keep other relevant conditions, such as water volume and stirring, the same; measure how quickly the salt dissolves; compare the results. Option A is wrong as the controlled factor because water temperature is the independent variable being changed. Option B is wrong because dissolving time is part of the outcome being measured, not the factor held constant in this wording. Option C is correct because equal salt amounts make the comparison fair. Option D is wrong because a result is not a factor that can be controlled before measurement. Memory cue: change one factor, measure its effect, hold the important others constant.