A student must determine how the mass of a block affects the period of oscillation when the block is attached to a vertical spring. The value of the spring constant is known. The student writes the following experimental procedure. Use an electronic balance to measure the mass of the block. Attach the block to the vertical spring. Displace the block from the system’s equilibrium position to a new vertical position. Release the block from rest. Use a meterstick to measure the vertical displacement of the center of mass of the block from the system’s equilibrium position to its maximum vertical position above the equilibrium position. Use a stopwatch to measure the time it takes for the system to make ten complete oscillations. Repeat the experiment for different vertical displacements and block masses. Which of the following steps of the procedure should the student revise to make the determination? justify your selection.