$4\,x^2 \,- 9 = $   ?

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

$49\,x^2 \,- 9\,y^2 = $   ?

$(7x+9y)(7x-9y)$ $(49x+3y)(49x-3y)$ $(7x+3y)(7x-3y)$ $(49x+9y)(49x-9y)$

$\,x^2 \,- 64\,y^2 = $   ?

$(\,x^2+8\,y^2)(\,x^2-8\,y^2)$ $(x+8y)(x-8y)$ $(x+8y)^2$ $(x-8y)^2$

$16\,y^2 \,- 64 = $   ?

$(16y+64)(16y-64)$ $(4y+8)(4y-8)$ $(4y+64)(4y-64)$ $(4\,y^2+8)(4\,y^2-8)$