John of Worcester, an English monk, recorded the sighting, on December 8, 1128, of two unusually large sunspots. Five days later a brilliant aurora borealis (northern lights) was observed in southern Korea. Sunspot activity is typically followed by the appearance of an aurora borealis, after a span of time that averages five days. Thus, the Korean sighting helps to confirm John of Worcester' s sighting. Which one of the following, if true, most strengthens the argument? ['Chinese sources recorded the sighting of sunspots more than 1000 years before John of Worcester did.', 'An aurora borealis can sometimes occur even when there has been no significant sunspot activity in the previous week.', 'Only heavy sunspot activity could have resulted in an aurora borealis viewable at a latitude as low as that of Korea.', 'Because it is impossible to view sunspots with the naked eye under typical daylight conditions, the sighting recorded by John of Worcester would have taken place under unusual weather conditions such as fog or thin clouds.']