4. The Mundell-Fleming model takes the world interest rate r* as an exogenous variable. Let's consider what happens when this variable changes. a. What might cause the world interest rate to rise? (Hint: The world is a closed economy.) b. If the economy has a floating exchange rate, what happens to aggregate income, the exchange rate, and the trade balance when the world interest rate rises? c. If the economy has a fixed exchange rate, what happens to aggregate income, the exchange rate, and the trade balance when the world interest rate rises