What Evidence Did Oskar Pfungst Present to Indicate That Clever

What evidence did Oskar Pfungst present to indicate that Clever Hans was not doing mathematical calculations?

A) Hans was correct no more often than we would expect for chance guessing.
B) Hans could answer a question correctly only if the questioner knew the correct answer.
C) Hans could answer correctly only if the questioner made subtle sounds while he was answering.
D) Hans could answer correctly only if his owner, Mr. von Osten, was present.