On the reading section. of the TOEFL, one of the type of reading questions is the inference question. An inference question requires to make an assumption, drawn a conclusion, based on the information given to you in the passage.

Example

The seasonal nature of agriculture in ancient Egypt, along with advances in food storage, meant that the people had more time for leisure. At least the wealthy classes had time for leisure; the lower classes became a reliable source of labor. Within this organized system with an abundance of food resources, a rich culture flourished. While the pyramids and the elaborate tombs of the pharaohs are the most well-known manifestation of this culture, there are countless other achievements, including calendars, a form of writing, sophisticated architecture (particularly of temples), and a complex belief system including a pantheon of gods.

What can be inferred from paragraph 4 about the development of sophisticated culture in the history of ancient peoples?

It was only possible in areas with distinct seasons.
It began in the lower classes and percolated up to the higher classes.
It depends on stability in food resources.
It is delayed when the people spend too much time on leisure activities.

The following strategy will help you choose the correct answer choice for this inference question above.

Strategy: Use the keywords
A. Identify the key words in the paragraph and underline them. I highlighted, in red, the key points of the paragraph above.
B. Using the keywords, highlighted in red in the paragraph above, to come up with a main idea for the paragraph.
C. Now that you have used those keywords to come up with a main idea, now looks at the answer choice and eliminate the ones that don’t match up with your main idea. Identify the answer choices that matches or comes close to matching your main idea. That is your answer.

Here is a model of another inference question already done for you. The keywords I highlighted in red.

C is the only one which comes close to your main idea,