Why Did President Richard Nixon Fire Archibald Cox

Why did President Richard Nixon fire Archibald Cox?

A) Cox had asked Judge Sirica to order Nixon to provide him with the tapes.
B) Cox had forced the testimony of Alexander Butterfield about Oval Office recordings.
C) Cox had pressured John Dean to testify with the threat of lifelong sentence.
D) Cox had leaked details about the Watergate investigation to the Washington Post.