If a, b, c, and d are the successive angles of a cyclic quadrilateral, then cos(a) * cos(c) * cos(c) * cos(b) equals to ________
a)cos(a+c)⋅cos(b+d)
b)cos(a)⋅cos(b)
c)cos²(c).cos²(d)
d)1/4.cos(2a)⋅cos(2b)