Read the Following Argument and Use It to Answer Question

Read the following argument and use it to answer question
There is every reason to believe that the concussion crisis in NFL football will get worse. The speed and size of professional athletes have made American football more dangerous. Offensive linemen now average nearly 315 pounds-65 more pounds than they did 30 years ago. They launch that weight from a three-point stance, headfirst, at opposing linemen of nearly the same size. The impact of two such massive players produces a concussion in the cranium when the brain bangs against the skull. Professor of Neurology at Boston University, Dr. Ann McKee, notes, "when helmets clash, the head decelerates instantly, yet the brain continues to lurch forward, like a driver who jams the brakes on. Even though the skull is protected by a helmet, the brain itself is still in motion and a concussion is often the result."
-In assessing the reliability of Dr. McKee's statement we would be concerned about

A) the recency of the observation.
B) the comparability of units being compared.
C) the expertise of the source.
D) the generalizability of settings and subjects.