Telephone numbers are preceded by a 3-digit area code (e.g. 437). 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 must not be 6? b. the first digit cannot be 0 or 1, the second digit cannot be 3 or 9, and the third digit must be 9? c. the first digit cannot be 0 or 1, the second digit must be 3 or 6, and the third digit must be 9? d. the digit 3 must not be used?