$81\,y^2 \,- 9\,x^2 = $   ?

$(9y-3x)^2$ $(9\,y^2+3\,x^2)(9\,y^2-3\,x^2)$ $(81y+9x)(81y-9x)$ $(9y+3x)(9y-3x)$

$100 \,- 25\,y^2 = $   ?

$(10+5y)(10-5y)$ $(10+25y)(10-25y)$ $(10+5y)^2$ $(100+25y)(100-25y)$

$121\,x^2 \,- 100 = $   ?

$(11x+100)(11x-100)$ $(11x+10)^2$ $(121x+10)(121x-10)$ $(11x+10)(11x-10)$

$144\,y^2 \,- 4\,x^2 = $   ?

$(144y+4x)(144y-4x)$ $(12y+2x)^2$ $(12y-2x)^2$ $(12y+2x)(12y-2x)$