https://learnenglish.britishcouncil.org/free-resources/grammar/english-grammar-reference/comparative-superlative-adverbs (theory and exercises)
https://www.eflnet.com/grammar/compsupadv1
https://www.ego4u.com/en/cram-up/grammar/adjectives-adverbs/adverbs/exercises?02
https://www.learnenglishfeelgood.com/adverbs-comparative-superlative1.html
