Which is the correct comparative or superlative adverb?

Question

Of the two basketball teams, Kate's team played harder or hardest?

When comparing her with one other person (so that two people are being compared), use the -er word.

When comparing her with two or more other people (so that three or more people are being compared), use the -est word.

http://grammar.ccc.commnet.edu/grammar/adjectives.htm
Scroll down to Degrees of Adjectives.

http://www.lousywriter.com/adjectives_comparison.php

http://www.uottawa.ca/academic/arts/writcent/hypergrammar/compsupl.html

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