If f(x)=ax+bcx+df(x) = \frac{ax+b}{cx+d}f(x)=cx+dax+b and f(f(x))=xf(f(x)) = xf(f(x))=x, which of the following must be true?
a=da = da=d
a=−da = -da=−d
b=cb = cb=c
a2+bc=0a^2 + bc = 0a2+bc=0