An oil field prospector and developer reported a large oil

For assumption questions, a great technique is something called the Negation Test. I'll teach it to you right now.

The concept rests on the information that the argument depends on the assumption. So how would you test dependence with something else, such as a table with legs? You'd remove a leg. If the table falls down, the table depended on that leg. If it doesn't fall down, it didn't depend on it. Same concept here. Negate (make negative) each answer choice. If that negation were true, it should make the argument's conclusion fall apart.

Let's look at this argument.

Step 1: Construct your table by listing the conclusion and premises of this argument.

Premise: The company found that what had been reported to be a large oil deposit was actually much smaller than had been indicated.
Assumption: ?
Conclusion: Thus, the methods that the prospector had used to determine the size of the oil deposit must have been inaccurate.

Step 2: Negate each answer choice:

Answer A:
The company's methods of measuring the size of the oil deposit were determined by a third party to be more accurate than those used by the prospector.
Think: What if the company's methods were NOT determined by a third party?
Result: So what? The prospector may still have been inaccurate. Eliminate.

Answer B: The prospector did not purposefully fabricate or misrepresent the size of the oil deposit.
Think: What if the prospector DID purposefully fabricate or misrepresent?
Result: Winner. If he lied, then maybe his results were NOT inaccurate at all. This is your answer. (Often, the right answer on an assumption question will contain the word "not".)

Answer C: Though smaller than originally thought, the oil deposit contained enough oil to make drilling commercially feasible.
Think: What if it did NOT contain enough oil to be commercially feasible?
Result: So what? The prospector may still have been inaccurate. Eliminate.

Answer D: The prospector did not explore other oil fields and use the same methods to determine the magnitude of the oil present, if any.
Think: What if the prospector DID explore other oil fields and use the same method?
Result: So what? The prospector may still have been inaccurate every time. Eliminate.

Answer E: The company had successfully drilled for oil in other large oil fields in Texas throughout the early twentieth century.
Think: What if the company did NOT successfully drill in other fields?
Result: So what? The prospector may still have been inaccurate.

You can use the negation test on any assumption question on the GMAT, whether it appears as a critical reasoning question in the Verbal, in the reading comp section, or in the IR section.

Best of luck in your GMAT study.