$49\,x^2 \,- 25\,y^2 = $   ?

$(7\,x^2+5\,y^2)(7\,x^2-5\,y^2)$ $(7x+5y)(7x-5y)$ $(7x+25y)(7x-25y)$ $(49x+25y)(49x-25y)$

$81\,x^2 \,- 36\,y^2 = $   ?

$(9x+36y)(9x-36y)$ $(81x+36y)(81x-36y)$ $(9x+6y)(9x-6y)$ $(9x+6y)^2$

$100\,y^2 \,- 16 = $   ?

$(10y+4)(10y-4)$ $(10\,y^2+4)(10\,y^2-4)$ $(100y+16)(100y-16)$ $(10y+16)(10y-16)$

$25 \,- 49\,x^2 = $   ?

$(5+49x)(5-49x)$ $(5+7x)(5-7x)$ $(5-7x)^2$ $(5+7x)^2$