Consider making a four-digit I.D. number using the digits 3, 5, 8, and 0.
a. How many I.D. numbers can be formed using each digit once?
b. How many can be formed using each digit once and not using 0 first?
c. How many can be formed if repetition is allowed and any digit can be first?
d. How many can be formed if repetition is allowed but 0 is not used first?