Adam was riding in a car driven by Natalia, whose reckless driving caused an accident, injuring Natalia but not Adam. After the accident, Adam accompanied Natalia to the hospital in an ambulance. The ambulance was subsequently hit by a car driven by Harry, injuring Adam. Adam filed a suit against Natalia, alleging that her negligence caused his injury. The element of this negligence claim that would be the most difficult to determine is

A. Proximate cause

B. The duty of care

C. Causation in fact

D. The injury requirement