In Recent Years, a New Currency Was Introduced

In recent years, a new currency was introduced. The primary reason was

A) to make the currency more difficult to counterfeit (technological advances had made counterfeiting easier) .
B) to increase the supply of currency in a growing economy.
C) to improve the appearance of the U.S. currency which was much less attractive than some foreign currencies.
D) to recall old dollars that were held in large amounts outside the United States.