$100\,x^2 \,- 25 = $   ?

$(10\,x^2+5)(10\,x^2-5)$ $(10x+5)(10x-5)$ $(10x-5)^2$ $(100x+25)(100x-25)$

$36 \,- 16\,x^2 = $   ?

$(6+4\,x^2)(6-4\,x^2)$ $(6+4x)^2$ $(6+16x)(6-16x)$ $(6+4x)(6-4x)$

$\,x^2 \,- 121 = $   ?

$(x-11)^2$ $(x+121)(x-121)$ $(\,x^2+11)(\,x^2-11)$ $(x+11)(x-11)$

$81\,x^2 \,- 144\,y^2 = $   ?

$(9x+12y)(9x-12y)$ $(9x-12y)^2$ $(81x+12y)(81x-12y)$ $(9\,x^2+12\,y^2)(9\,x^2-12\,y^2)$