But it is the only source of water, 可是这是唯一的水源

19. The air is just too cold and too dry for it to do so.
    又冷又干的空气使它们无法解冻
20. The sun's rays turn it straight into vapour.
    阳光把它们直接变成了水蒸气
21. It evaporates.
    它们升华了
22. But it is the only source of water,
    可是这是唯一的水源
23. so Bactrian camels eat it.
    所以双峰驼只能吃雪
24. Elsewhere in the world
    在世界上其它地方
25. a camel at a waterhole can drink as much as 200 litres during a single visit.
    一只骆驼找到水坑后 能一次喝下200升水

来自《行星地球 第一季 第五集 Planet Earth S01E05 沙漠》第 22 行