With a relative pronoun: With the police's persistent efforts, a

With a relative pronoun: With the police's persistent efforts, all______ had been stolen was returned to its rightful owner.
【正确答案】：that
【题目解析】：定语从句中，当先行词是下列不定代词或被它们修饰时much, little, none, all, few, every(thing), any(thing), no(thing) 等时，通常用that。