Although the geological record contains some hints of major meteor impacts preceding mass extinctions, there were many extinctions that did not follow any known major meteor impacts. Likewise, there are many records of major meteor impacts that do not seem to have been followed by mass extinctions. Thus the geological record suggests that there is no consistent causal link between major meteor impacts and mass extinctions. Which one of the following assumptions enables the argument's conclusion to be properly inferred? ['If there were a consistent causal link between major meteor impacts and mass extinctions, then all major meteor impacts would be followed by mass extinctions.', 'Of the mass extinctions that did not follow any known major meteor impacts, few if any followed major meteor impacts of which the geological record contains no hints.', 'There could be a consistent causal link between major meteor impacts and mass extinctions even if not every major meteor impact has been followed by a mass extinction.', 'Major meteor impacts and mass extinctions cannot be consistently causally linked unless many mass extinctions have followed major meteor impacts.']