Astudent is investigating the affect of different salts on melting points. four patches of ice of equal size are roped off and a different type of salt is poured on each, one receives table salt (nacl), one receives calcium chloride (cacl2), one receives potassium carbonate (kco3) and the fourth receives inert sand instead. each patch receives an equal amount of salt or sand. the student measures the volume of ice remaining and subtracts it from the original volume of ice to see how much melted away. what is a control variable in this experiment?