Hailey pretests a group of participants to determine their attitudes toward the use of nuclear power sources to replace fossil fuels. Hailey then initiates a 2-month program to convince them to invest in nuclear power. During this time, the price of coal rises 100% per pound. At the end of the program, Hailey retests and finds that her participants are much more likely to endorse nuclear power over coal. What potential threat to internal validity most likely accounts for this change? a. testing b. regression toward the mean c. maturation d. history