By chance, you may come across a GRE reading comprehension passage whose main topic you are familiar with, or maybe even whose main topic you know quite a bit about. Or, you might come across a topic which you may have come across in a class or while reading a book or a newspaper. Whatever the case, be sure that when answering GRE reading comprehension questions, you use only the information that is in the passage -- do not use outside knowledge!
If you refer back to the instructions for the GRE reading comprehension questions:
Each of the following reading comprehension questions is based on the content of the following passage. Read the passage and then determine the best answer choice for each question. Base your choice on what this passage states directly or implies, not on any information you may have gained elsewhere
notice that you are explicitly told to answer each question based on the information that is provided IN the passage, else information that is implied. Save your factual knowledge for the issue essay, instead, where you can showcase your knowledge in support of the claims that you are trying to make.
How can you use this to your advantage? Often times several of the answers to a reading comprehension question may be correct, but which are not explicitly stated in the passage, in which case you must eliminate them. Reading comprehension questions test your ability to read critically, to infer, and to make implications from the material that you are provided; reading passages are NOT meant to test your knowledge or memorization skills that you may have learned elsewhere.
When answering reading comprehension questions, do not use outside knowledge. There may even be answer choices which otherwise are correct and valid but for which supporting information was not included in the reading passage. Any answer that cannot be supported with information from the reading passage must be eliminated -- don't be fooled!