$25 \,- 4\,y^2 = $   ?

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

$\,y^2 \,- 64\,x^2 = $   ?

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

$\,y^2 \,- 81 = $   ?

$(y+9)^2$ $(y-9)^2$ $(\,y^2+9)(\,y^2-9)$ $(y+9)(y-9)$

$9\,x^2 \,- 1 = $   ?

$(3x+1)^2$ $(3x+1)(3x-1)$ $(9x+1)(9x-1)$ $(3\,x^2+1)(3\,x^2-1)$