A point in space $(x,y,z)$ is randomly selected so that $-1\le x \le 1$,$-1\le y \le 1$,$-1\le z \le 1$. What is the probability that $x^2+y^2+z^2\le 1$?
(A)\frac{\pi}{24} (B)\frac{\pi}{12} (C)\frac{\pi}{6} (D)\frac{\pi}{3}
C