According to the context the term that adequately completes the fragment is Herd immunity.
<h3>What is herd immunity?</h3>
Herd immunity is a term to refer to a biostatistical phenomenon that originates in a population when most of it has become immune to a disease (due to previous infection or because it has been vaccinated) and the epidemiological chain is interrupted, causing an indirect form of protection against a disease by preventing unimmunized individuals from becoming infected.
According to the above, the appropriate term to complete the fragment would be herd immunity.
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It is an opinion because it is just someone's view, not a fact that it is better
I think it’s c sorry if I am incorrect
It required all territories to abolish slavery before becoming states.
And it made the mississippi river the divding line between slave states and free states. it also prohibited slavery in most of the former louisiana territory except for missouri.