A psychologist has developed a set of activities intended to help children develop better reading skills. In a study of the effectiveness of these activities, four classes of second grade children in a public school were invited to participate and the activities were conducted by their respective teachers. Two of the classes (A and B) learn with the activities. The other two classes (C and D) serve as the control and learn without the activities. After some period of time, the reading skills of all of these children were assessed. A summary of these data is: