Yes, extensively is an adverb. It means to a wide or
comprehensive degree.
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The word extensively is an adverb. It means to do something in
an extensive manner.
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Venus was the planet that the spacecraft Magellan enabled
scientists to research extensively.
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The Tana and Arthi are Kenyas largest rivers. The are both
extensively exploited for Hydro electric power which account for
54% of installed capacity in the country.