U.S. stocks staged their biggest rally this month after Standard & Poor’s kept AAA debt ratings for the nation’s largest bond insurers, easing concern credit losses will extend the worst earnings slump since 2001. The S&P 500 advanced 18.69 points, or 1.4 percent, to 1,371.8. The Dow Jones Industrial Average rose 189.2, or 1.5 percent, to 12,570.22. The Nasdaq Composite Index added 24.13, or 1.1 percent, to 2,327.48. Almost six stocks climbed for every one that fell on the New York Stock Exchange.