Outside a​ home, there is aa 44​-key keypad with letters Upper A comma Upper B comma Upper C comma and Upper DA, B, C, and D that can be used to open the garage if the correct fourfour​-letter code is...

Outside a​ home, there is

aa

44​-key

keypad with letters

Upper A comma Upper B comma Upper C comma and Upper DA, B, C, and D

that can be used to open the garage if the correct

fourfour​-letter

code is entered. Each key may be used only once. How many codes are​ possible?