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Ir has not been common at Earth’s since the very beginning of the planet’s history. Because it usually exists in a metallic state, it was preferentially incorporated in Earth’s core as the planet cooled and consolidated. lr is found in high concentrations in some meteorites, in which the solar system’s original chemical composition is preserved. Even today, microscopic meteorites continually bombard Earth, falling on both land and sea. By measuring how many of these meteorites fall to Earth over a given period of time, scientists can estimate how long it might have taken to deposit the observed amount of lr in the boundary clay. These calculations suggest that a period of about one million years would have been required. However, other reliable evidence suggests that the deposition of the boundary clay could not have taken one million years. So the unusually high concentration of lr seems to require a special explanation.
13. Look at the four squares that indicate where the following sentence could be added to the passage.
Consequently, the idea that the lr in the boundary clay came from microscopic meteorites cannot be accepted.
Where would the sentence best fit?
14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
For a long time scientists have argued that the extinction of The dinosaurs was related to climate change.
Answer choices
1. A simple climate change does not explain some important data related to the extinction of the dinosaurs at the end of the Cretaceous.
2. The retreat of the seaways at the end of the Cretaceous has not been fully explained.
3. The abruptness of extinctions at the end of the Cretaceous and the high concentration of lr found in clay deposited at that time have fueled the development of a new hypothesis.
4. Extreme changes in daily and seasonal climates preceded the retreat of the seas back into the major ocean basins.
5. Some scientists hypothesize that the extinction of the dinosaurs resulted from the effects of an asteroid collision with Earth.
6. Boundary clay layers like the one between the Mesozoic and Cenozoic are used by scientists to determine the rate at which an extinct species declined.