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. 1 These calculations suggest that a period of about one million years would have been required. 2 However, other reliable evidence suggests that the deposition of the boundary clay could not have taken one million years. 3 So the unusually high concentration of lr seems to require a special explanation. 4 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?