Your environmental science professor designs an experiment to see whether students in her class care about recycling. She places a recycling bin and a trash bin at the front of the class and then hands out a piece of paper to every student. She asks them to write on the piece of paper their student number, their favorite movie, and whether they own a pet. At the end of the class, she asks all of the students to dispose of the paper before they leave. Then, the professor retrieves the papers from either the trash bin or recycling and counts how many papers ended up in each bin. In this experiment, what variable is measured?
A. How many students listed the same favorite movie.
B. How many students had pets.
C. How many students cared about the environment.
D. How many papers were recycled or thrown in the trash.