Which statement best expresses the central idea of the passage?
Read an Informational Passage · ELPAC Grades 6–8 reading. The correct answer, and the reasoning behind it.
A longer factual text, with questions about the main idea, supporting details and vocabulary.
Read the passage. Then answer the questions.
The Rocks That Move by Themselves
On a dry lakebed in Death Valley National Park called Racetrack Playa, hundreds of rocks sit at the ends of long, shallow trails. Some trails run straight for hundreds of feet; others curve or zigzag. Some of the rocks weigh more than a hundred pounds. For decades, nobody had ever seen one move.
People offered explanations of every kind. Some suggested powerful winds; others proposed that sheets of ice had dragged the rocks, or that the ground itself shifted. A few blamed pranksters. Each idea had a weakness. Winds strong enough to push a heavy rock across dry ground would have to be far stronger than any ever measured there, and no one could explain how pranksters would leave no footprints in the soft mud.
In the early 2010s, a team of researchers decided to stop guessing and start watching. They placed rocks fitted with GPS trackers on the playa, set up a weather station and time-lapse cameras, and waited. Then, one winter, it happened. A shallow layer of rainwater had collected on the playa and frozen overnight into ice only a few millimeters thick. As the morning sun warmed it, the ice broke into large, thin panels. A light breeze pushed the floating panels, and the panels pushed the rocks, which slid slowly through the mud beneath the water and carved new trails.
The answer turned out to combine pieces of several older ideas: ice, wind and water all played a part. But none of those ideas had been right on its own, and it took patient observation, not clever arguments, to show how the pieces fit together.
The question
Choose the best answer.
Which statement best expresses the central idea of the passage?
- Careful observation finally showed that thin ice, wind and water together move the rocks on Racetrack Playa.Correct answer
- Some of the rocks on Racetrack Playa weigh more than a hundred pounds.
- For many years, people believed pranksters were moving the rocks.
- Strong desert winds are the only force that can push heavy rocks across dry ground.
Why “Careful observation finally showed that thin ice, wind and water together move the rocks on Racetrack Playa.” is correct
A central idea has to cover the whole passage: the mystery, the failed explanations, the observation that solved it, and the conclusion about how it was solved. Only the correct choice includes both the answer (ice, wind and water together) and how it was found (patient watching). The other choices are details from one paragraph or ideas the passage rejects.
Why the other answers are wrong
- Some of the rocks on Racetrack Playa weigh more than a hundred pounds. — The weight of the rocks is a detail from the first paragraph that shows why the movement was puzzling.
- For many years, people believed pranksters were moving the rocks. — Pranksters were one explanation "a few" people offered, and the passage gives a reason to doubt it.
- Strong desert winds are the only force that can push heavy rocks across dry ground. — The passage says winds that strong were never measured there, and that wind alone was not the answer.
Practice more of this
More reading practice for Grades 6–8, or see the ELPAC practice test for grades 6–8. For other grades, see free ELPAC practice tests by grade.
This is an independent study resource. It is not affiliated with, endorsed by, or connected to the California Department of Education or the official ELPAC program. All questions here are original practice material written to match the published task formats.