Some people fear that global warming will cause the large ice formations in the polar seas to melt, thereby warming the waters of those seas and threatening the plankton that is crucial to the marine food chain. Some scientists contend that it is unlikely that the melting process has begun, since water temperatures in the polar seas are the same today as they were a century ago. Which one of the following, if true, most seriously undermines the scientists'contention? ['The overall effect of the melting process will be an increase in global sea levels.', 'The mean temperature of ocean waters near the equator has remained constant over the past 100 years.', 'The mean air temperature above both land and water in the polar regions has not varied significantly over the past 100 years.', 'The temperature of water that contains melting ice tends to remain constant until all of the ice in the ice-and-water mixture has melted.']