Task: Question Answering
Input (English): Students run laps around the track in physical education class. After a few laps, the students' breathing gets heavier, and their pace begins to slow. The students are most likely slowing down because  
 (A) they are exhaling carbon dioxide. (B) their lung cells are multiplying. (C) their respiratory system stops functioning. (D) they are getting less oxygen to their cells.
Output (English): D