+ Complete the text below using suitable passives. Where a modal passive is required, be careful with the tense choice: In 1990, ten years after Nobel prizewinner Luis Alvarez first suggested a link between the extinction of the dinosaurs and an asteroid collision, a huge impact crater _______________ (discover) underground, near the village of Chicxulub in the Yucatan, Mexico. Recent research has shown that this crater _______________ (could not + cause) by volcanic activity. This is because a significant quantity of the element iridium, which is extremely rare on earth, _______________ (detect) at the crater site, supporting the view that the crater _______________ (must + form) by an asteroid. 65 million years ago, iridium-enriched dust _______________ (would + throw) up into the atmosphere, causing the sun _______________ (blot out) for many weeks. As a result, the temperature would have plummeted and the plants which _______________ (eat) by the dinosaurs _______________ (would + kill) by the accompanying acid rain. With the removal of their food chain, the dinosaurs could not have survived.