National bank regulate the nation's economy by controlling the supply of currency (option A). For example, when some countries face an economic crisis, the National bank has "printed" more money. The problem with this measure is that the money loses its value, so the prices escalate quickly. This situation is seen in Venezuela, the actual government issued more money, but didn't apply other strategies to face the crisis. Thus, people have thousands of bills, but they are worth nothing, or almost nothing.