Omar wants to determine if the mass of a model rocket affects how long the rocket is able to stay up in the air. To do this, he constructs three identical rockets and then fills two of the rockets with varying amounts of sand to add mass. He then launches the rockets one at a time and times how long they are able to stay airborne. What is the test variable (independent variable) in Omar's experiment? A. the masses of the model rockets B. the force with which each rocket is launched C. the materials out of which the rockets were made D. the time the rockets remain airborne