One historian writes that the Code of Hammurabi was the most important source of modern law. Another historian argues that modern law is based on the legal system of the Roman Republic. What could be the reason for the difference of opinion between these experts?

one historian did not examine primary sources

one of the historians' research has serious flaws

they used different secondary sources for research

their interpretations of historical facts vary

I would say the fourth one: Their interpretations of historical facts vary.

freedom of speech is also protected by the 1 amendment.

explanation:

here is pretty much what the 1st amendment says:

congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.