A phrase is considered as a small group of words standing together as a conceptual unit, typically forming a component of a clause.
The two commas in the last sentence serve:-
Correct option is C.
The author uses the two commas to separate three comments, introduced by “But you will tell me,” that his audience could make to counter his earlier argument about nineteenth-century artists’ relationship to their surroundings.
By using commas this way, the author is able to quickly identify and distinguish among three related claims within a potential counterargument to his own views.
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