Several days into the study, some of the students in the math-based reading program were getting frustrated by the frequent questions from their teacher. The students wanted to see the story in the books progress quickly, and they began acting out. The teacher decided it was best to either let these students join the control reading program or to try reading on their own in a different part of the classroom. Thus these students no longer participated in the math-based reading program. The researchers found that students in the math-based reading program performed much better on the PICO post-test than students in the control reading program. Which internal validity threat is most likely occurring in this scenario? a. Maturation
b. Regression to the mean
c. Selection
d. Mortality
e. Instrumentation
f. Testing
g. History
h. Interactions
i. Diffusion
j. No Threat