Glad that helped, Sandy! And I'm not surprised that that was the mistake you made - what's neat about the GMAT is that you can start to learn how they toy with your mind to get you to make those assumptions. Particularly on Data Sufficiency be really careful with those assumptions and always be skeptical about which knowledge you truly do have. I've always seen it this way - there are really only two ways that they can elicit a wrong answer from you on DS:
1) You think you have enough information, but you actually don't
-this usually comes in the form of an assumption you make (that x is an integer, or positive, or you forget to consider 0)
-it also comes when you prematurely archive the information in statement 1 and use it on statement 2. But you don't know the information from statement 1 when you're looking at "Statement 2 ALONE" so be careful.
-like you saw here, they can also mention a word elsewhere in the question (prime, even, etc.) and get you to start thinking only about that type of number. So be careful with that - if the statement deals with x and y and says that "y is even", make sure that you note "y = even; x = unknown" or something to keep each variable (and what you know about each) straight.
2) You think you don't have enough information, but you actually do
-this can happen when the situation calls for an integer or a positive number (a number of children is always an integer, as you can't have a fractional child; the length of a side of a triangle has to be positive because it can't be negative or 0))
-you should also check for this any time that the answer C seems obvious. Is there a way to derive the information from statement 1 while only using statement 2? They won't too frequently hand you C on a silver platter, so if it looks like they're doing that be careful. This article should give some more information on that:
https://www.beatthegmat.com/mba/2010/12/ ... knight-way
As you study, pay attention to the ways they trap you like this and you'll start to look out for them proactively. It's subtle but pretty powerful the way they bait your mind toward thinking only in one direction...
