Essentially all polar ice forms from precipitation that falls as snow. Extremely cold air cannot hold much moisture and consequently cannot produce much snowfall. In recent years, air masses in both polar regions have been, without exception, extremely cold. The information above most strongly supports which of the following conclusions? ['If air temperatures in the polar regions were considerably warmer, much polar ice would melt off.', 'The thicker the polar ice is, the colder the air masses that are in contact with it.', 'For snow to turn into ice in the polar regions, the air has to be extremely cold.', 'If polar ice is currently growing and expanding at all, it is doing so only slowly.']