I am not sure how to approach this

Case: New Instructional App

High school math teacher Mr. Gualtieri begins his class on Monday with an important announcement: "Our school has just purchased a new instructional app that we can use on our classroom tablet computers. This app, called Problem-Excel, will give you practice in applying the mathematical concepts and procedures we'll be studying this year. I strongly encourage you to use it whenever you have free time so that you can get extra instruction and practice with things you might be having trouble with."

Mr. Gualtieri is firmly convinced that the new app will help his students better understand and apply certain concepts in his math curriculum this year. To test his hypothesis, he keeps a record of which students use the app and which students do not. He then looks at how well the two groups of students perform on his next classroom test. Much to his surprise, he discovers that, on average, the students who have used the app have earned lower scores than those who have not used it. "How can this be?" he puzzles. "Is the app actually doing more harm than good?"

Mr. Gualtieri wonders whether the instructional app is actually hurting, rather than helping, his students. Assume that the app has been carefully designed by an experienced educator. Assume, too, that Mr. Gualtieri's classroom test is a good measure of how well his students have learned the material they've been studying.

A. Explain why Mr. Gualtieri cannot draw a conclusion about a cause-and-effect relationship from the evidence he has. Base your response on principles of educational research. B. Identify another plausible explanation for the results Mr. Gualtieri has obtained.