The banking system currently has $100 billion of reserves, none of which are excess. People hold only deposits and no currency, and the reserve requirement is 10 percent. If the Fed lowers the reserve requirement to 5 percent and at the same time buys $10 billion worth of bonds, then by how much does the money supply change?

a. It rises by $200 billion.

b. It rises by $800 billion.

c. It rises by $1,200 billion.

d. None of the above is correct.

Which is correct?