A student was asked to estimate the concentration of reducing sugar in an unknown solution using the Benedict's test. Five reducing sugar solutions with different concentrations were provided in order to produce a calibration curve. The student added 2 cm³ of Benedict's solution to each of the reducing sugar solutions, heated them in a water-bath and recorded the time taken for the first appearance of a colour change. Which variables should the student standardise, when carrying out the Benedict's test on each reducing sugar solution, to ensure the results are comparable? 1 volume of reducing sugar used 2 the temperature of the water-bath 3 the time the solutions are heated