To defend the (indigent) criminal to the best of their ability - the same as any defense attorney.
Answer
B. Spell checker
Explanation
Spell checker is a program designed to identify misspelled words and notify the writer about the mistake. The programs can further auto correct the word or allow the writer to choose from various corrections of the word. In Microsoft, a red underline appears on the misspelled word to display the error.
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Synonyms for the word externality could be: face, area, level, side, skin, top, etc.