Gresham's law in Economics shows that "bad money drives out good money". This means that when two types of money are in circulation, people will tend to hoard the money that they believe is of higher value and spend the money that they believe is of lower value. In other words, people will try to get rid of the "bad" money and hold onto the "good" money. This can lead to a situation where the "bad" money becomes the dominant currency in circulation, while the "good" money is hoarded or taken out of circulation. This phenomenon can occur when there is a difference in the intrinsic value of the two types of money, or when there is a difference in the perceived value of the two types of money. Therefore, the correct option is that "bad money drives out good money" is what Gresham's law in Economics shows.