对于小的、简单的树，从工程代价上来说使用XML： Simple通常更低廉。
For smaller and simpler trees, using XML: : simple is generally cheaper in terms of engineering costs.
阿波罗工程中使用的所有飞船都不能重复使用，令其实施起来代价高昂。
All of the crafts used on the Apollomissions were single-use, making the Apollo missions extremely expensive.
没错，任何碳排放定价方案都需要付出我们付出相应的代价，终结气候变化并不是一项免费工程。
It's true that there will be costs associated with any carbon-pricing plan; ending climate change won't be free.
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