Which of the following statements is (are) correct? The Federal Reserve

a. can, over the long run, roughly control the money supply by changing the monetary base to offset any undesirable changes in the money stock as a result of changes in currency holdings or excess reserve holdings.
b. controls the money supply better in the long-run because of short-run uncertainty regarding the money multiplier.
c. is in absolute control of the money stock at all times.
d. Both a and b