For an arc AB of a circle with centre O, C is any point on the circle different from A and B. Which relation between the central angle ∠AOB and the angle ∠ACB at the circumference is always true?
(a) ∠AOB=∠ACB
(b) ∠AOB+∠ACB=180∘
(c) ∠AOB=2∠ACB
(d) 2∠AOB=∠ACB