$\,x^2 \,- 1 = $   ?

$(x+1)^2$ $(\,x^2+1)(\,x^2-1)$ $(x-1)^2$ $(x+1)(x-1)$

$81\,y^2 \,- 1 = $   ?

$(9\,y^2+1)(9\,y^2-1)$ $(81y+1)(81y-1)$ $(9y+1)(9y-1)$ $(9y-1)^2$

$9 \,- 81\,y^2 = $   ?

$(3+81y)(3-81y)$ $(3+9y)(3-9y)$ $(9+81y)(9-81y)$ $(3-9y)^2$

$121 \,- 64\,y^2 = $   ?

$(121+8y)(121-8y)$ $(11+8y)(11-8y)$ $(121+64y)(121-64y)$ $(11+8\,y^2)(11-8\,y^2)$