But their mother must keep them out and active. 可是母熊必须得把它们带到外边活动

50. and spring is advancing rapidly.
    春色匆匆
51. But even now blizzards can strike without warning.
    但即使是现在 暴风雪 仍会在毫无征兆的情况下来袭
52. Being so small, the cubs are easily chilled and they will be more comfortable resting in the den.
    幼小的熊崽很容易冻僵 躲在洞穴中可能会更舒服些
53. But their mother must keep them out and active.
    可是母熊必须得把它们带到外边活动
54. She's becoming weak from hunger
    饥饿将它折磨得虚弱不堪
55. and there's no food on these nursery slopes.
    而这片产仔的斜坡上没有一点食物
56. The sea ice still holds firm,
    海面上的冰层依然坚固

来自《行星地球 第一季 第一集 Planet Earth S01E01 从北极到南极》第 53 行