学习外语时，习语常是其中最难学的部分。

When studying a foreign language, idioms are often the most difficult thing to learn.

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尽管如此，写作一直被认为是其中最难的一项。

However, writing is considered as the most difficult one.

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化妆店的东西数以百计，而其中最难挑选莫过于化妆刷。

Of all hundreds of items to choose from at a cosmetics store, the brush section may be the most difficult to navigate.

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