极端寒冷的天气和氧气过少是两个主要的困难。

Terribly cold weather and too little oxygen were two main difficulties.

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一个主要的困难是让机器人知道它们的定位。

One key difficulty is getting robots to know their place.

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一个主要的困难是评估这种使用工具的行为是否是智能的标志。

A major difficulty is assessing whether this tool-using behaviour is a sign of intelligence.

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And kids were having a hard time in school just because of the language barrier.

在校的儿童们遇到的困难主要来自于语言沟通障碍。

帮助多文化的家庭 - SpeakingMax英语口语达人

I think that's the main difficulty.

我想这是主要的困难。

巴西的英语老师 - SpeakingMax英语口语达人