If dimensions of critical velocity `v_c` of a liquid flowing through a tube are expressed as `[eta^x rho^y r^z]`, where `eta, rho and r` are the coefficient of viscosity of liquid, density of liquid and radius of the tube respectively, then the values of `x, y and z` are given by
Options:
(a) `1-1-1`
(b) `-1,-1,1`
(c) `-1, -1,-1`
(d) `1,1,1`