For the purposes of illustration, let's take a look at #17 from the first Verbal section of the GRE PowerPrep Test #2.
Question 17 is based on the following passage.
Extensive housing construction is underway in Pataska Forest, the habitat of a large population of deer. Because deer feed at the edges of forests, these deer will be attracted to the spaces alongside the new roads being cut through Pataska Forest to serve the new residential areas. Consequently, once the housing is occupied, the annual number of the forest's deer hit by cars will be much higher than before construction started.
17) Which of the following is an assumption on which the argument depends?
A) The number of deer hit by commercial vehicles will not increase significantly when the housing is occupied.
Explanation: Identifying the assumption of an argument is often closely related to finding alternate explanations for a phenomenon. An argument consists of two main elements: 1) the premise (better known as “evidence”) and 2) the conclusion. The premise in this case is that deer tend to feed on the edges of forests, and that the extensive construction of a new housing development will create more such edges. The conclusion is that the number of deer hit by cars will drastically increase.
In-between the premise and the argument lies the inference (better known as “reasoning”), that which connects the premise to the conclusion in a convincing way.
In real life, you can attack an argument by disputing the validity of the premise itself (“that’s not true,” “your data is incorrect/flawed”, etc.), but on the GRE (and the GMAT) that’s not an option (all presented facts are assumed to be true). Thus, you must attack the argument by attacking the inference.
The correct answer is Choice D.