The same side of the Moon is always toward the Earth, but more than half of its surface has been seen due to libration. Libration in latitude occurs because __________.

A
the axis of rotation is tilted about 6.5° to the axis of revolution
B
the speed of revolution varies, while the rotational speed is essentially constant
C
of the rotational oscillation of the Moon with respect to its radius vector
D
of augmentation
E