作为同位语的名词短语中用a还是the?

经常会见到一个名词短语对前一个词进行解释说明，这便是同位语。请看下列句子：

We have two foreign teachers, Canadian and __ American.

We have two foreign teachers, a Canadian and an American.

我有2个外教老师，他们是谁呢？逗号后面的部分进行解释说明，一个加拿大人一个美国人

2. The word "Mid-Autumn" first appeared in the famous ancient book Zhou Li[The Zhou Rituals, _ book ______(tell) the rituals in the Zhou Dynasty].

[]里面的整个部分对Zhou Li进行说明，而[]里面的book对The Zhou Rituals，应用a，表示一本怎样的书，后面的内容对book进行修饰，分词作后置定语，根据book和tell的关系应用telling

3. A line from Xin Qiji, __ poet during the Song Dynasty, shows this, "Restlessly, I searched for her in thousands, hundreds of ways. Suddenly I turned, and there she was in the receding light."

A line from Xin Qiji, a poet during the Song Dynasty, shows this, "Restlessly, I searched for her in thousands, hundreds of ways. Suddenly I turned, and there she was in the receding light."[/toggle]

BTW, "Restlessly, I searched for her in thousands, hundreds of ways. Suddenly I turned, and there she was in the receding light."对应辛弃疾的那句词句？

众里寻他千百度，蓦然回首，那人却在灯火阑珊处。

TO conclude, 作为同位语的名词短语中常用a还是the?

作为同位语的名词短语中冠词常用a