Which of these is one of the biggest differences between Roman and Greek art? A. Roman artists preferred mythological themes. B. Roman artists focused on landscape painting. C. Roman artists made realistic depictions of their rulers. D. Roman artists used mathematical perspective.

The question is asking for the biggest difference between Roman and Greek art. Option A is incorrect because both Roman and Greek artists used mythological themes in their art. Option B is also incorrect because landscape painting was not a significant focus in either Roman or Greek art. Option D is incorrect because mathematical perspective was not a significant difference between Roman and Greek art. Option C, however, is correct. Roman artists were known for their realistic depictions of their rulers, which was a significant difference from Greek art.