Keep in mind that background images only show in the available "doorway" of the element to which they're applied (content area + padding).

在头脑中，始终要有这样的认识：背景图像只是显示一个可供内容填充的有效空间(即内容区域和padding，称为doorway)。

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As lead and arsenic pollution element are skew distribution, statistics of a natural background value and a content of lead and arsenic are worked out using median method of skew distribution.

由手铅、砷污染元素呈偏态分布，故采用偏态分布中的中位数法作了铅、砷自然背景值及含量的统计。

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First, as a rigid element to help frame the page and separate the content areas from the background.

首先，作为一个硬性元素，以帮助框架页和脱离背景的内容领域。

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