Mary works in a top secret research laboratory. in order to gain access to the building, she must use a secret four-digit code. all codes from 0000 to 9999 are possible. as a security feature, mary is required to get exactly one digit wrong each time, and not to enter the same four-digit number as she did last time. the security guard can see the numbers that she enters into the keypad. what is the maximum number of different four-digit numbers mary can enter without the security guard being able to be certain of the secret code? a2 b8 c 32 d 4096