that is (to say)
The navy was planning an expedition to the South Pacific Ocean with the purpose of watching a very unusual event, that is the planet Venus passing between the earth and the sun in 1769. (《高中英语教科书》第三册，人教社，p.8) 我们学校的许多英语老师都认为上句中 that 应当改为 which。因为 that 是不能引导非限制性定语从句的。但是，重庆的曹毅老师在《英语辅导报》(高中教师版)上就上句发表文章说，that 可以引导非限制性定语从句，并举了大量的实例。这使我感到无所适从。
你们学校的老师以及重庆的曹老师均误解了原句的结构。
首先要明白, that is the planet...并不是一个定语从句, 因而 that 不是关系代词, 谈不上用 which 替换的问题, 也谈不上 that 从句能不能用于非限制性定语从句的问题。
其实这里的 that is = that is to say(这就是说,亦即)。其后的 the planet Venus passing between the earth and the sun in 1769 是 a very unusual event 的同位语。
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John is a New Yorker; that is he lives in New York. 约翰是纽约人, 这就是说, 他住在纽约。
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We're leaving on 2 March, that is, Friday. 我们预计3月2日，即星期五，动身。
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I'll meet you in the city, that is, I will if the trains are running. 我将在城里与你会面，那就是说，只要火车还在运行，我就会见到你。
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He's a local government administrator, that is to say a Civil Servant. 他是地方行政官员，也就是说，是文官。
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The fare is reduced for children, that is, anyone under 15 years old. 儿童, 即凡是年龄在十五岁以下者, 票价有优惠。
[附注]在正式的书面语中，that is (to say)前后通常要加逗号。
还有一点必须指出, passing 并不是现在分词作 the planet Venus 的定语, 而是动名词, the planet Venus 是它的逻辑主语。
[句意]海军计划到南太平洋进行考察,目的是观察一个不平常的事件,即金星在1769年在地球与太阳之间通过的景观。
请做下面的英语试题：
Codes are a way of writing something secret; ____, anyone who doesn't know the code will not be able to read it.
A. that is B. worse still C. in short D. on the other hand(全国六级,90.1-42)