Debbie is interested in determining if music helps people study. To do this, she uses the help of 40 students in her psychology class. She randomly assigns half the people to study with music in the background for an hour, and the other half study with no music for an hour. The students then take a test on that material. In this example, what is the independent variable? 
选项为:A:Music versus no music  B:Group that studies with no music  C:Group that studies with music  D:~Scores on the exam  E:Students in the class  请回答其中一个选项.