Its livelier because that compares only two things. Thanks so much for the opportunity to answer your question. I hope this helps! :)
The best choice to complete the sentence is 'livelier', as it is the correct comparative form for describing the students' cheering in a comparison between two groups. This choice adheres to the grammatical rules of comparatives. Thus, the completed sentence reads: 'The students were cheering livelier in the stands than they were on the field to celebrate the championship.'
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