The only info we get from the question stem is that Set T is an infinite supersequence; we don't know whether it is arithmetic or geometric sequence (or something else). We are asked whether there are a finite number of multiples of 3.
(1) The first six integers in T are multiples of three.
The first six are integers are multiples of 3, so from (1), we know there are some multiples of 3 in the set. But we don't know whether or not multiples of 3 continuously recur in Set T. Insufficient.
(2) An infinite number of integers in T are multiples of four.
This statement tells us that there are an infinite number of multiples of four. Some multiples of four are multiples of three; namely, every third multiple of 4 is also a multiple of 3 (ie, 4, 8, 12, 16, 20, 24...). However, knowing that Set T contains an infinite number of multiples of 4 does not tell us EVERY multiple of 4 is in the set. (We can't just assume that the multiples of 4 are consecutive). Thus, we don't know whether every third (positive) multiple of 4 is included in the set. Insufficient.
(1) + (2)
Combined, you still don't know whether every third multiple of 4 is included in the set.
The statements are insufficient even when combined.
Choose E.
Kaplan Teacher in Toronto
