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5. [20 pts.] An empirical model of U.S. net exports to GDP (NX/GDP in left axis) is assumed to be a function of the real exchange rate (RER), which is the trade-weighted broad dollar index (TWEXBPA in right axis) between the U.S. and several countries: NX = f (RER). The graph between the two variables is reproduced below. Answer the following questions: 0.020 140.000 0.010 120.000 0.000 -0.010 0.020 -0.030 1973-01-01 1975-05-01 1977-09-01 1980-01-01 1982 05-01 1984 09 01 19870101 1989-05-0 1991-09-01 1994-01- 1996-05-01 1998 09 01 2001 01 01 2003 05 01 2005 09:01 2008 01-01 2010 05 01 2012 09:01 100.000 80.000 60.000 0.040 40.000 -0.050 -0.060 20.000 -0.070 0.000 NX/GDP TWEXBPA (a) Explain the expected effect (sign) of RER on NX based on economic theory. Note that an increase in RER in this graph means a real appreciation of the U.S. dollar versus the basket of other currencies. (10) (b) Modify the model above for NX, choosing appropriately one more explanatory variable (X1) to be used jointly with RER. Explain the expected signs in your new net export function: NX = f(RER, X1): "when X1 goes up, NX goes up (or down) because ...."