In a word, the occurrence of leftover children is not caused by a single factor.
总之，留守儿童的出现并不是由单个因素造成的。
In data base terminology, a single instance, for example, a set occurrence, a record occurrence.
在数据库术语中，一个单一实例，例如，一组具体值，一个记录的具体值。
In data base terminology, a single instance, for example, a set occurrence, a record occurrence.
在数据库术语中，一个单一实例，例如，一组具体值，一个记录的具体值。
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