Residents of the tiny island of Yeonpyeong waited anxiously to see if North Korea would carry out its threat to resume shelling. But the North said the exercises were not worth reacting to.South Korea on Monday conducted an hourlong live-fire artillery drill on an island at the center of recent tensions on the Korean peninsula, but North Korea, which had pledged to attack the South if the exercises took place, later said that it would not retaliate.