Find the angle between any two diagonals of a cube.

Question

$(a)\:\;cos^{-1} 1/3\:\:\:\qquad\:(b)\:\:cos^{-1}1/\sqrt 3\:\:\:\qquad\:(c)\:\:cos^{-1}2/3\:\:\:\qquad\:(d)\:\:cos^{-1}1/3 \sqrt 3$