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なんです, なのです
1. it is assuredly that ..., can say with confidence that ...
〜んです句型;可以肯定地说……,可以有把握地说……
は
1. indicates sentence topic
2. indicates contrast with another option (stated or unstated)
3. adds emphasis
指示句子的主题;表示与另一选项(无论是否明确提及)形成对比;表示强调