$121\,x^2 \,- 49\,y^2 = $   ?

$(121x+7y)(121x-7y)$ $(11x+49y)(11x-49y)$ $(11x+7y)(11x-7y)$ $(11x+7y)^2$

$49 \,- \,x^2 = $   ?

$(7+x)(7-x)$ $(7+\,x^2)(7-\,x^2)$ $(7+x)^2$ $(49+x)(49-x)$

$9 \,- 4\,x^2 = $   ?

$(3+4x)(3-4x)$ $(3+2\,x^2)(3-2\,x^2)$ $(9+2x)(9-2x)$ $(3+2x)(3-2x)$

$64\,x^2 \,- 25\,y^2 = $   ?

$(8x-5y)^2$ $(8x+5y)(8x-5y)$ $(8x+25y)(8x-25y)$ $(8x+5y)^2$