21.) What two criteria must be met before scientists can use radiocarbon dating?

22.) Describe the meteorite collision at the end of the Mesozoic affected the atmosphere and the biosphere.

21. There must be enough sample to test properly. 22. Asteroid impact may be a better term than meteorite collision. The impact made the atmosphere opaquer by filling it with dust, and indirectly, smoke, from fires touched off by hot impact debris. Since the atmosphere admitted much less sunlight, photosynthesis and plant productivity were curtailed, with disastrous results, especially for those with the greatest energy requirements, the dinosaurs. Of course, many dinosaurs were killed directly by the impact in areas nearest the Yucatan zero e. g. North America