During the Pandemic, the means for transportation is really very difficult, so Pepe bought a bicycle and uses it as his service vehicle to go to his place of work. Unluckily, he forgot the 4-digit...

During the Pandemic, the means for transportation is really very difficult, so Pepe bought a bicycle and uses it
as his service vehicle to go to his place of work. Unluckily, he forgot the 4-digit lock code of the chain he uses
to secure his bicycle. If the code is arranged in a linear pattern, and he only remembers that it has the digits 2,
5, 6, and 9, then
(a) List all the possible codes.
(b) How many codes are there in all?