单选题 The passage you are reading is the beginning part of a report in the original. Then, what is "This", the first word, most probably referring to?

【正确答案】 C

【答案解析】[分析] 推断题型。第一段第一句的This is offered as a textbook illustration of the principle...(这个可作为……原则的文本例证)，显然是一个引出下文的句子。