某生物興趣小組為了探究蚯蚓生活環境,做了如下實驗:將一個長方形的塑料盒用不透水的隔板分為三格,隔板高度為塑料盒的三分之二,在甲中放入土壤并用水淹沒土壤,乙中加入疏松濕潤土壤,丙中加入疏松干燥土壤,然后,在甲、乙、丙上分別放10條大小和生長狀況相同的蚯蚓,放在適宜的環境中觀察現象。請回答下列問題:
(1)該實驗的變量是
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(2)放入蚯蚓后觀察到的現象是
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出現這種現象的原因是
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(3)該實驗的結論是
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