又遇到了一个经济学难题，百思不得其解。请大家帮忙给出推导过程，非常感谢！

A country’s government runs a budget surplus of $10 billion. What must the country’s central bank do to prevent cash reserves of the commercial banks from falling?
A buy bonds of a value at least equal to $10 billion
B buy bonds of a value less than $10 billion
C sell bonds of a value at least equal to $10 billion
D sell bonds of a value less than $10 billion