Identify the independent variable in the research scenario described below. 

A grad student designs a thesis project that looks at music's effect on exhaustion during maximal exercise. Fifty participants visit the lab and cycle to fatigue while listening to high-tempo music of their choice. Their time to exhaustion is recorded. 

Two weeks later, the same group of people return to the lab and cycle to exhaustion without music. Their time to exhaustion is recorded. 

Later, the grad student compares the times to exhaustion to see if there are any differences. 

What is the independent variable?

Select one:

a. The cycling session

b. Time to fatigue

c. The thesis hypothesis 

d. Music usage