$16\,y^2 \,- 1 = $   ?

$(4y-1)^2$ $(4\,y^2+1)(4\,y^2-1)$ $(16y+1)(16y-1)$ $(4y+1)(4y-1)$

$144\,y^2 \,- 121\,x^2 = $   ?

$(12\,y^2+11\,x^2)(12\,y^2-11\,x^2)$ $(12y+11x)(12y-11x)$ $(12y+11x)^2$ $(144y+121x)(144y-121x)$

$64\,x^2 \,- 64 = $   ?

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

$1 \,- 81\,y^2 = $   ?

$(1-9y)^2$ $(1+81y)(1-81y)$ $(1+9y)(1-9y)$ $(1+9y)^2$