It happens occasionally that a student who is in one of our classes has previously taken the class from another instructor. The repeater students will comment that the class is so much easier with us as instructors

Now, we might want to believe that we are such great instructors that student learning (the dependent variable) has drastically improved because of us (the independent variable). However, which threat to internal validity better explains the increase in student learning?

a. attrition
b. testing
c. diffusion of treatment
d. instrumentation

B