托福阅读句子插入题20

　　托福阅读中句子插入题是重要题型之一。根据托福阅读题型不同来进行专项练习，是短期提高托福阅读能力的有效方法。同学们需要多加练习哦!

　　托福阅读句子插入题：Petroleum Resources

　　【Paragraph 2】Continued sedimentation—the process of deposits’ settling on the sea bottom—buries the organic　　matter and subjects it to higher temperatures and pressures, which convert the organic matter to oil and gas. ■As muddy　sediments are pressed together, the gas and small droplets of oil may be squeezed out of the mud and may move into　sandy layers nearby. ■Over long periods of time (millions of years), accumulations of gas and oil can collect in the sandy　layers. ■Both oil and gas are less dense than water, so they generally tend to rise upward through water-saturated rock and sediment. ■(Petroleum Resources)

　　Look at the four squares [■] that indicate where the following sentence could be added to the passage.

　　Unless something acts to halt this migration, these natural resources will eventually reach the surface.

　　Where would the sentence best fit?