Answer to Question 1
(a) Selectiondifferent types of students may take the two different classes, so they may differ in their ratings of psychology at the start.
(b) Attritiondifferent types of students may drop the two courses, especially if one course is required and they have to stick with it.
(c) Historysomething in one class, like an instructor's change in grading policies may affect students positively or negatively.
Answer to Question 2
Symmetric transfer takes place when condition B, following condition A is affected the same way as A is after following B. In asymmetric transfer, the effect of B on A is different than the effect of A on B.