A researcher investigates attitudes toward recycling. She first gives a pre-test and plans to have participants come to the lab to watch a short film on the benefits of recycling. Following the film, she plans to measure their recycling attitudes again. Before she can play the film, however, the campus holds a recycling drive. What threat to internal validity does this represent?

A. Testing
B. History
C. Instrumentation
D. Maturation
E. Selection