A researcher would like to compare two methods for teaching math to third-grade students. Two third-grade classes are obtained for the study. Mr. Jones teaches one class using method A, and Mrs. Smith teaches the other class using method B. At the end of the year, the students from the method-B class have significantly higher scores on a mathematics achievement test. Does this result indicate that method B causes higher scores than method A? Explain your answer, and identify the general problem that precludes a cause-and-effect explanation.