The correct answer to this open question is the following.
If a singer's song transcends musical genres, you don't classify them as either rap or rock. This statement includes a part of the answer when it says "transcends musical genres." If that is the case it means that the song is a now a "classic" beyond a broader term call rock, that originally was written under the influence of a musical genre or the fusion of different rhythms. But with the pass of time or popularity, the song is part of the culture of a generation that is considered that trascended musical genres and simply is popular.