專四英語選擇題The atmosphere is as much a part of the earth as _ it

題目：

專四英語選擇題
The atmosphere is as much a part of the earth as _ its soils and the water of its lakes ,rivers and oceans.
請解釋下爲什麼選擇填are,並分析說明一下.

解答：

這種比較狀語從句常省略主語
現在省略了地球上的其他部分 所以複數