There's nothing like a GRE reading comprehension inference question to bedevil the assiduous test-taker. Without enough experience with official ETS material, it can be difficult to judge whether a particular inference is justified by the passage, or whether it's too speculative.
I break inference questions down into two types:
1. "Must be true". For example, if a reading passage says that most phytoplankton species are found at the bottom of a deep-sea canyon, a correct answer choice could be that "relatively few phytoplankton species are found at the rim of the canyon".
2. "Would be silly if untrue". If you're not sure about a choice but you'd feel like a doofus if you tried to construct an argument against it, it's almost certainly correct.
Pro tip: It's important to calibrate your understanding of inference questions by practicing with the official ETS GRE prep books, as well as the ETS Big Book (the old PDF with 27 tests).