Official ExplanationIt may be easier to think about this problem with smaller numbers. For example, if set S contained the multiples of 3—3, 6, 9, 12, 15, 18 ...—and set T contained the multiples of 5—5, 10, 15, 20, 25 ...—then the intersection, or overlap, of the two sets would be the set of all multiples of 15. This set—15, 30, 45, 60 ...—continues indefinitely and is infinite. In general, if one set consists of all multiples of m and another set consists of all multiples of n, then the intersection of the two sets will contain all multiples of the integer, mn, which is an infinite set, as long as neither number is 0. Infinity is greater than 9, so
(A) is correct.
Answer: A