GMAT Data Sufficiency (DS) part is something that definitely needs your time and pre-understanding. OG has a quick intro into these types of questions, however, it wasn't something that I was able to get right away.
My first block of tests brought me to 50% of correct rate, which is extremely low.
However, taking some time and going through the answers with explanations, I was able to run next two blocks with 80% of success. While it's still far from heaven, it's way better I expected to see from the first run.
Hint: it was very helpful for me to draw an algorithm of solving these questions. There is one in the book as well, but I preferred the one I made myself. It's all about the order.
Step 1. If 1 is sufficient ? (Yes) Answers could be AD, if (No) BCE.
Step 2. If 2 is sufficient ? (Yes) Answer is D, (No) is A.
Step 3. If 1 is not sufficient, is 2 ? (Yes) Answer is B, otherwise (No) answers could be CE.
Step 4. If 1 & 2 together sufficient ? (Yes) Answer is C, if (No) is E.
This could look awkward, but it's not. I was able to save a lot of time by memorizing such a simple scheme.