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【解析】：Meeting my uncle after all these years was a happy moment,【one】( that ) I will always remember.这么多年以后,和我叔叔的会面是美好的一刻,我将会永远铭记在心.
这里的 one = a moment ,表示泛指.在这里相当于做 I will always remember 的先行词,括号里的 that 作定语从句里的 remember 的宾语成分,可以省略.one I will always remember 总体来说的话,是做句子的同位语.that 不可以引导非限制性定语从句,故是错误的.如果去掉逗号, 就不用one 了,用 which 和 that 都可以.即是：
Meeting my uncle after all these years was a happy moment that / which I will always remember.这里的 that 和 which 是可以省略的.
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