研究人员说，最佳水平范围是每毫升30到40纳克。

Optimal levels range from 30 nanograms per milliliter to 40 nanograms per milliliter, the researchers said.

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哪些水平范围很大可以引起结果发生改变，但是结果依然可测量？

What levels are wide enough apart to induce a change to the response, but still make a measurable result?

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弹出汉语：这个天才的团队教授水平范围广泛，但是其他语种在线网站比较少。

Popup Chinese: This talented team teaches a wide range of levels, but has a niche that few other online language sites offer, preparation for the standardized Chinese language test known as HSK.

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