甲状腺激素有促进蝌蚪发育的作用。为你提供40只大小、发育状况相似的蝌蚪、两个水缸、水草、清水、不含任何激素的食物、甲状腺活性物质等材料。请你设计一个对照实验，来验证甲状腺激素的作用。
（1）实验变量：                                                                       。
（2）实验设计：                                                                                                                                  
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（3）实验结果：                                                                                                                     。
（4）得出结论：                                                                                。