Which is correct?
The _______ of the two drives was causing shoe-shining.
We need the comparative form here, faster, because we’re comparing two things. If we were talking about more than two drives, we’d use the superlative form, fastest.
Those are the rules for positive (fast), comparative (faster) and superlative (fastest) adverbs and adjectives.
Englishwizo has a handy table with more examples.
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