Suppose the federal government had budget deficits of $40 billion in year 1 and $50 billion in year 2 but had budget surpluses of $30 billion in year 3 and $50 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

• increased by $10 billion.
• increased by $90 billion.
• decreased by $80 billion.
• decreased by $10 billion.