Biff Bifferson wanted to test a new sing-along method to teach math to fourth graders (e.g., "Multiplying Is Fun" to the tune of Happy Birthday). He used the sing-along method in his first period class. His sixth period students continued solving math problems with the old method. At the end of the term, Mr. Bifferson found that the first period class scored significantly lower than the sixth period class on a mathematics achievement test. He concluded that his singalong method was a total failure. Which of the following is a method to "unconfound" the experiment? Group of answer choices Only use the sing-along method Counterbalance so that both classes experience both methods Use a song other than Happy Birthday Teach something other than multiplication