Please, explain to me why correct answer is C

[Olegoldie]

http://www.english-test.net/toefl/listening/A_lecture_by_a_professor_of_Art.html

2). Why do the Lascaux cave paintings probably NOT qualify as graffiti?
(A) They are prehistoric.
(B) They were painted quickly.
(C) They were painted by the cave's inhabitants. -- CORRECT
(D) Their artists did not sign their works.

[Beeesneees]

Here's the key sentence:

So defacing public property with graffiti has been a universally popular pastime since the world's earliest history.

The cave paintings were not public property.
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