Commentator: Many people argue that the release of chlorofluorocarbons into the atmosphere is harming humans by damaging the ozone layer, thus allowing increased amounts of ultraviolet radiation to reach Earth. But 300, 000 years ago a supernova greatly damaged the ozone layer, with no significant effect on our earliest ancestors. Because the supernova' s disruption was much greater than the estimated effect of chlorofluorocarbons today, there is no reason to think that these chemicals in the atmosphere harm humans in this way. Which one of the following, if true, would most seriously weaken the commentator's argument? ['The ozone layer regenerates at a slow rate, barring counteractive processes.', 'Our earliest ancestors possessed genetic characteristics making them more resistant than we are to the harmful effects of ultraviolet radiation.', 'Scientists have discovered that genetic changes occurred in our ancestors during the period in which the supernova affected Earth.', 'Extraterrestrial influences on the ozone layer tend to occur less often than terrestrial ones.']