What is most closely the meaning of ascetic as it is used in this passage from the text?

On a rooftop near O'Connell Bridge, a Republican sniper lay watching. Beside him lay his rifle and over his shoulders was slung a pair of field glasses. His face was the face of a student, thin and ascetic, but his eyes had the cold gleam of the fanatic. They were deep and thoughtful, the eyes of a man who is used to looking at death.

A. Concerned with beauty
B. Free from indulgence or pleasure
C. Religious
D. Solitery