Why is difficult to define, or rather discover and achieve happiness?

为何很难定义、发现和获得幸福？

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According to the passage, why is it difficult for foreigners to learn Chinese?

根据文章，为什么外国人学习汉语很难？

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Why is the obesity problem in Britain so difficult to solve?

为什么英国的肥胖问题如此难以解决？

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A newly released history of 20th Century Russia, written by a team of 45 scholars and edited by historian Andrei Zubov, includes a statement that much of the country's recent past is painfully difficult to study, which is why many refuse to do so.

VOA: standard.2009.12.25

But I want you guys to think about why it was difficult to do it another way, which is more pedestrian.

但是我希望你们能思考一下,为什么用另一种更普遍的方法来做,反而会比较难呢

耶鲁公开课 - 基础物理课程节选

Why is it difficult in some cases to produce a vaccine when we've been successful in so many others?

为什么某些疾病的疫苗很难研制,即使我们已经成功生产了很多种疫苗

耶鲁公开课 - 生物医学工程探索课程节选

That is why it is difficult to predict the outcome of these elections, and why when you talk to people in the streets, or go out in restaurants, or interview analysts, you will hear as many scenarios as there are people ready to talk.

CNN: View From Belgrade: Milosevic and the quest for power

And this is why it is difficult to see anyone reversing the decision any time soon.

ECONOMIST: The Hubble space telescope

That is why it is so difficult to study and why the studies show mixed results.

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