(A) Define the "Offer Curve" (Or "Reciprocal Demand Curve") of a Country

(a) Define the "offer curve" (or "reciprocal demand curve") of a country. If an offer curve is drawn as an upward-sloping curve, what is being assumed about the value of thecountry's elasticity of demand for imports and why does this assumption yield the upward-sloping curve?
(b) Using the usual two-good, two-country offer-curve diagram, identify the equilibrium position and state why the position is one of equilibrium. Then suppose that, from this initial equilibrium position, one country now experiences an increase in productivity in its export industry at the same time that the other country imposes an import tariff. Illustrate and explain the combined or overall impact of these two events on the equilibrium terms of trade and on the quantity traded of each of the two goods. If a combined impact is uncertain, briefly indicate why it is uncertain.