Final answer:
The text does not mention the tier placement of teachers for early COVID-19 vaccination, so the answer based on the provided text is 'C. Not mentioned.'
Step-by-step explanation:
Based on the provided chapters 14 and 16 from the textbook, the statement that "teachers are in the third tier of those eligible for early vaccination against COVID-19" is not directly referenced. Therefore, the correct answer to whether this statement is true, false, not mentioned, or varied by region cannot be determined using this text. It is possible that such information may have been decided at different levels of government or within different jurisdictions, suggesting that the answer could possibly be "D. Varied by region," if external sources were to be considered. However, without direct text support, the appropriate answer based solely on the provided text information is "C. Not mentioned."