Stumpf and His Student Oskar Pfungst Discovered That Clever Hans

Stumpf and his student Oskar Pfungst discovered that Clever Hans, "the counting horse," was able to perform so many arithmetic calculations because

A) his owner gave him subtle cues to indicate the correct answer
B) persons in the crowd watching his performances had been paid to cooperate
C) the sequence of correct numbers was always the same in each performance
D) the horse actually possessed some extrasensory abilities