夏天买回的豆腐,存放时间稍长就会变质。有同学提出，将豆腐放入冰箱中冷藏就能延长保鲜时间。小强为了验证这一方法，设计了如下实验：

根据以上资料回答问题：
（1）小强设计该实验的变量是____ 。
（2）设置乙实验的目的是____ 。
（3）导致豆腐腐败变酸的根本原因是____ 。

    Mr. Smith is my new friend. He1from Paris, France. He is a French2of a middle school. His students all like3very much. He can speak some Chinese,4not much. He wants to learn more, so he goes to5 classes every week. He likes living and working6China very much. He7 the Chinese people are very friendly(友好的). He has two8, Maria and Tom. They are students, but they go to different(不同的)9. They have some pen pals and they often write10 the pen pals. They also have many Chinese friends at school and they play games after class.