3. The sidebar "How the Grand Canyon Formed" states that "the area was covered in sand dunes" 275 million years ago. What can you infer happened to the dunes? (A) They turned into rock. (B) They were washed away by rivers. (C) They became part of a mountain. (D) They were completely blown away by wind

The correct answer is **(A) They turned into rock.** **Explanation:** Sand dunes composed of sand can undergo a process called **lithification**, where the loose sand is compacted and cemented over millions of years to form **sandstone**. This geological process is a key factor in the formation of many of the rock layers observed in the Grand Canyon today. The other options are less likely because: - **(B) Washed away by rivers**: If the dunes were entirely washed away, they wouldn't be present in the geological record. - **(C) Became part of a mountain**: Sand dunes typically form sedimentary rock, which is different from the processes that form mountains. - **(D) Completely blown away by wind**: Similar to option B, if the dunes were entirely removed, they wouldn't have been preserved to contribute to the Grand Canyon's structure. Therefore, the transformation of sand dunes into rock is the most plausible inference. **Answer:** (A) They turned into rock.