Great question, shimmer - keep in mind that Data Sufficiency is as much about logic as it is about math (if not more so), so you probably need to spend some time getting comfortable with the "game" of DS. The way that the questions are asked can give you some insight in to how to best answer them. For example, answer choice C says:
(C) BOTH statements together but NEITHER statement ALONE is sufficient.
Well, even though a lot of times it feels comforting to have both statements, you're not allowed to use both statements if one alone will get the job done. The GMAT will reward you for managing your resources efficiently - if you can answer the question without both statements, you need to do that. C is a great trap answer in many cases because you're conditioned to use all available information to solve a problem so a natural inclination is to start using everything you're given. But if you don't need statement 1 - if 2 alone is sufficient - you have to answer as such. So your strategy should be that, if you know pretty quickly that both together are sufficient, you have to try to get one alone to work.
There's obviously more to that but that's just one example - if you know the way that the DS game is played then you'll work much more effectively toward those answers. This article may help:
https://www.veritasprep.com/blog/2011/07 ... trategies/
