Final answer:
The passage demonstrates credibility through historical specifics, references to the Harvard University History Department and the British Museum, and exact periods.
Step-by-step explanation:
Words and phrases that show credibility in the passage about Ancient Rome include specific historical details, references to esteemed institutions, and precise timeframes. The mention of Rome being a republic for 450 years before becoming an empire under Julius Caesar, and the fall of the Roman Empire by the fifth century AD, provide a temporal context that supports the passage's credibility. Additionally, the passage includes credible sources like the blog maintained by the History Department at Harvard University and findings from the British Museum, both of which are respected institutions that lend authority to the information presented.