Suppose the federal government had budget surpluses of $80 billion in year 1 and $120 billion in year 2 but had budget deficits of $10 billion in year 3 and $40 billion in year 4. Also assume that it used its budget surpluses to pay down the public debt. At the end of these four years, the federal government's public debt would have:

decreased by $150 billion.

decreased by $200 billion.

increased by $150 billion.

increased by $50 billion.