$100\,x^2 \,- 4\,y^2 = $   ?

$(100x+2y)(100x-2y)$ $(10x+4y)(10x-4y)$ $(10x+2y)^2$ $(10x+2y)(10x-2y)$

$121 \,- 81\,y^2 = $   ?

$(11+9\,y^2)(11-9\,y^2)$ $(121+9y)(121-9y)$ $(11+9y)(11-9y)$ $(121+81y)(121-81y)$

$36\,x^2 \,- 121 = $   ?

$(6x+11)^2$ $(6x-11)^2$ $(6x+11)(6x-11)$ $(36x+121)(36x-121)$

$16\,y^2 \,- 100 = $   ?

$(4y+10)(4y-10)$ $(4y+10)^2$ $(4\,y^2+10)(4\,y^2-10)$ $(4y+100)(4y-100)$