The Volume of Oxygen Consumed (In Liters Per Minute) While

The volume of oxygen consumed (in liters per minute) while a person is at rest and while he or she is exercising (running on a treadmill) was measured for each of 50 subjects. The goal of this study was to determine whether the volume of oxygen consumed during aerobic exercise can be estimated from the amount consumed at rest. The results are plotted below.
What is the explanatory variable in this study?

A) Volume of oxygen consumed at rest
B) Volume of oxygen consumed while running
C) Volume consumed at rest or while running; it does not matter which variable is considered the response