Answer:
the third sentence
Step-by-step explanation:
The topic of a text is the subject of the text, or what it covers.
Topic
The topic of the paragraph is people that made sacrifices to better the world. This means to stay on topic the writer should only provide claims or evidence that directly shows a sacrifice that improved the lives of others. The third sentence does not discuss this topic, instead, it gives the author's belief.
Tone and Purpose
The text is informational and provides facts and examples to back up the claims in the text. Informational texts like this have a formal tone and tend to not include personal opinions. So, the opinion seen in the third sentence strays from the original tone and goes off-topic. Additionally, the purpose of this paragraph is to provide the readers with information about this topic. Thus, the third sentence does not fulfill this purpose.
For these reasons, the third sentence should be removed.