If the public debt increased by the same amount each month during the past three years, then

a. The US treasury must have issued securities to fund a flow of government spending that exceeded a flow of tax revenues by the same amount during each of the past three months

b. During each of the past three months, the US treasury must have bought back the same amount of securities that had previously been issued to cover deficits experienced more than three months ago

c. The US government must have operated with the same budget surpluses during the past three months

d. The US government must have experienced budget surpluses that increased by the same amount each of the past three months