Let a two-output cost function (as a function of outputs) be given by: $C(y_1, y_2) = y_1 + y_2 + 2y_1y_2 - (y_1y_2)^2 + y_2^3$, where $y_1$ and $y_2$ are (positive) output levels. This cost function exhibits economies of scope:
(a) for all $y_1 > 0$, $y_2 > 0$
(b) for all $y_1y_2 > 1$
(c) for all $y_1 > 1$
(d) for all $y_1y_2 > 2$