Agroup of students is investigating whether silver is a better thermal conductor than copper. the students take a silver wire and a copper wire of the same length and diameter. they put equal lengths of wax on one end of each wire and dip the other end into a beaker of hot water. the length of wax left on the wires after 10 minutes is shown. experimental observations silver copper original length of wax 3.5 cm 3.5 cm length of wax after 10 minutes 1.2 cm 2 cm what was a control variable in this experiment?