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