Use the following table to answer the next question. the money supply and investment are in billions. money supply (billions of dollars), interest rate, investment (billions of dollars) (respectively listed): [$50 - 7% -$100], [60 - 6 - 110], [70 - 5 - 120], [80 - 4 -130], [90 - 3 -140]. assume that the mpc is 0.9 and the reserve requirement is 0.2. if the federal reserve needs to increase aggregate demand by $100 billion at each price level to move the economy back to full employment and the current interest rate is 6%, then the federal reserve should the money supply by (a) buy, $2 billion (b) sell, $4 billion (c) buy, $4 billion (d) sell, $2 billion