According to paragraph 4, what did Sir Gilbert Walker discover? A. There is a close link between El Niño and the Southern Oscillation. B. Surface pressure readings all across the Pacific first rise and then fall before an El Niño occurs. C. Surface pressure on one side of the Pacific tends to fall when pressure rises on the opposite side. D. The formation of an El Niño depends on conditions all across the Pacific, not just in the waters off of South America.