Stone, wood, tree bark, clay, and sand are generally available materials.
石材，木材，树皮，泥土，沙，一般可用的材料。
It occurs because less oxygen is available in the clay than in the sand.
产生腐蚀的原因是粘土内的氧含量较砂土内的少。
The bottom layer is a mixture of sand and clay.
最下面的一层是沙和泥的混合物。
The total amount of air space in clay is somewhat greater than in sand;
黏土中的气隙的总量在某种程度上要比砂土中的大。
填满了泥土或砂砾的。
填满了泥土或砂砾的。
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