$4\,x^2 \,- 25\,y^2 = $   ?

$(2x+5y)^2$ $(2x+25y)(2x-25y)$ $(2x+5y)(2x-5y)$ $(4x+5y)(4x-5y)$

$49\,y^2 \,- 121\,x^2 = $   ?

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

$121\,y^2 \,- 121\,x^2 = $   ?

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

$49\,x^2 \,- 36 = $   ?

$(7x+6)^2$ $(7x+6)(7x-6)$ $(7x-6)^2$ $(49x+36)(49x-36)$