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 condition must hold?
a=da = da=d
a+d=0a + d = 0a+d=0
a=−ba = -ba=−b
c=0c = 0c=0