Question 15 E 12 pts '01 G) Details Telephone numbers are preceded by a 3-digit area code (e.g. 902}. In the following questions, any digit from {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} can be used unless otherwise stated. How many area codes are possible if: a. the first digit cannot be 0 or 1, the second digit cannot be 1 or 6, and the third digit must be 6? b. 0 or 1 must not be used in any of the three digits? c. the first digit cannot be 0 or 1, the second digit must be 5 or 6, and the third digit must not be 0? l l d. the first digit must be 5 or 6, the second digit must be 0 or 1, and the third digit must not be 0? l l