Which best compares AC and DC?

a
AC flows in one direction, and DC repeatedly switches direction.
b
AC is used only in generators, and DC is used only in motors.
c
DC is used only in generators, and AC is used only in motors.
d
DC flows in one direction, and AC repeatedly switches direction.
Please Help me, I have no clue what I'm reading, and I'm trying to make a good grade on this formative assesement