Elementary, Intermediate, Upper Intermediate English.
初级，中级，中高级英语。
It consists of the following layers: the upper homogenous layer, with uneven external and internal surfaces, the intermediate homogenous layer, the alveolate layer, and the lower homogenous one.
外壁由几层组成：上部均质层，具不平的外、内表面，中间均质层，穴状层和下部均质层。
It consists of the following layers: the upper homogenous layer, with uneven external and internal surfaces, the intermediate homogenous layer, the alveolate layer, and the lower homogenous one.
外壁由几层组成：上部均质层，具不平的外、内表面，中间均质层，穴状层和下部均质层。
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