为控制氮污染，无论过去还是现在，我们都做出了许多尝试。 但我们认为最大的挑战还是将现行的政策领域联系起来使它们共同发挥作用。
There have been and still are many attempts to control nitrogen but we believe the big challenge is to link existing policy areas and make them work together.
关于汞含量的新闻强调了制定联邦政策控制污染排放的重要性。
The news about mercury also underscores the importance of developing a federal policy to control emissions.
这项政策的实施是为了控制环境污染，因为塑料袋造成的污染已经成为都市环境的一大威胁。
This policy is carried out for controlling pollution since the pollution resulting from plastic bags has become a big threat to urban environment.
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