I am pretty sure the answer is
D) Velocity
Glad I could help, and good luck!
AnonymousGiantsFan
Answer: Experiment.
Explanation:
Suppose that we have two related variables, A (independent) and B (dependent)
We could design an experiment where we can manipulate the value of A at will, and we also can observe how the variable B changes.
This is called an experiment, where the objective is to see how the variable B is related to the variable A, and then try to make a model that explains this relationship.
A scientific theory changes when there is more information available about what the theory covers.