Three-digit positive integers such as 789 and 998 use no digits other than 7,8 and 9 . In total, how many three-digit positive integers use no digits other than 7,8 and 9 ?
(A) 36
(B) 6
(C) 9
(D) 18
(E) 27
For each of the three digits of such a positive integer, there are three choices of digit $(7,8$ or 9$)$. Therefore, there are $3 \cdot 3 \cdot 3=27$ possible integers that use no digits other than 7,8 or 9 . (We note that there are 9 such integers beginning with each of 7,8 and 9 . The 9 such integers beginning with 7 are $777,778,779,787,788,789,797,798,799$.