C If dimensions of critical velocity v of a liquid flowing through a tube are expressed as [n*pr*], where n, p 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 : (a)-1,-1,1 (c) 1, 1, 1 (b) 1,-1,-1 (d) 1,-1,-1