很高兴为你解答.
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正确答案是：B
【解析】：
It was in the small town that /which he used to live in that he found his missing dog
这里面包括一个强调句型（ It is / was ...that）和一个定语从句.
第一空是作定语从句里面的宾语成分,故用 that/which都可以.第二空是强调句型里面的 that.
如果句子里面 live in 的in去掉,就可以选C.
【翻译】：
就是在他以前住的小镇里,他找到了他丢失的狗狗.
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