(b) Yes, this is the better argument. These facts illustrate the importance of distinguishing the legal concept of causation from other concepts. It might be scientifically or philosophically true to say that if A was not present at the scene of the collision the harm would not have occurred. However, a court is only interested in negligent conduct that contributes to the harm. Since the facts clearly show that A was not acting in breach of any duty of care, her conduct is not a factor that contributes causally to the harm she has suffered.
B's conduct was clearly negligent, but this alone is not enough to conclude that it caused the harm. It must be shown that the harm was the result of B's negligent conduct. The most basic and useful test that can be applied to decide this is the 'but for' test. Ask: 'Would the harm have occurred but for the defendant's conduct?' If it would not have occurred, it can be concluded that the defendant's conduct was a necessary condition of the harm.
Applied in this case, the test would indicate that B's negligent conduct was the cause of A's injuries.