Which Statement Is an Example of "Philosophic Doubt

Which statement is an example of "philosophic doubt"?

A) Attention is the most powerful reinforcer for most clients so it will probably work with Freddie.
B) I have many reservations about using the principles of behavior in designing treatment programs.
C) I really do not think that food functions as a reinforcer for Linda since she often misses meals.
D) Past research has shown that verbal behavior training is effective but this could change when new research is conducted.