Landing on the moon's far side is extremely challenging.

登陆月球的背面是极具挑战性的。

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Reproducing these conditions on Earth is extremely challenging.

在地球上重现这一反应是一个极大的挑战。

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He said the past two years have been extremely challenging for China.

他说，过去两年对中国来说是极其困难。

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It was done over "extremely challenging" terrain, officers said, including mountains, gorges, streams and waterfalls.

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It can be extremely challenging and it requires a tremendous amount of self-awareness, action and accountability.

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Mr Colley described market conditions in the UK as "extremely challenging" and difficult to predict.

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