Professor Robinson: A large meteorite impact crater in a certain region was thought to be the clue to explaining the mass extinction of plant and animal species that occurred at the end of the Mesozoic era. However, the crystalline structure of rocks recovered at the site indicates that the impact that formed this crater was not the culprit. When molten rocks crystallize, they display the polarity of Earth' s magnetic field at that time. But the recrystallized rocks recovered at the site display normal magnetic polarity, even though Earth' s magnetic field was reversed at the time of the mass extinction. Each of the following is an assumption on which Professor Robinson' s argument depends EXCEPT: ['The crater indicates an impact of more than sufficient size to have caused the mass extinction.', 'No other event caused the rocks to melt after the impact formed the crater.', 'The recovered rocks melted as a result of the impact that formed the crater.', 'The recovered rocks recrystallized shortly after they melted.']