Why are artists such as Vincent van Gogh and Paul Gauguin sometimes called avant-garde?

a)  They pioneered new and creative approaches to art that were often ahead of the public’s ability to comprehend.
b)  They had once served as foot soldiers in the French military.
c)  They believed that art should elevate the poor and working classes.
d)  They lacked formal artistic training and acceptance among their peers.

a)  They pioneered new and creative approaches to art that were often ahead of the public’s ability to comprehend.