If `alpha`,`beta``gamma` are the roots of the equation `x^3+px^2+qx+r=0`, then `(1-alpha^2)(1-beta^2)(1-gamma^2)` is equal to
(a)   `(1+q)^2-(p-r)^2`   (b)   `(1+q)^2+(p+r)^2`
(c)   `(1-q)^2+(p-r)^2`   (d)   none of these