What Is a Reason It Might Be Hard for the Fed

What is a reason it might be hard for the Fed to restore aggregate demand in the face of a negative demand shock?

A) The Fed might run out of money.
B) Banks usually don't do what the Fed demands of them.
C) The Fed must operate in real time,when a lot of the data about the state of the economy are unknown.
D) The economy responds to the Fed's actions with no lag.