1 . What is the coefficient of $ x^{12}y^{12} $ in the expansion of $ {(3x-7y)}^{24} $?
(a)
$-3^{12}7^{12}{{24}\choose{12}}$
(b)
$(3x)^{12}(7y)^{12}{{24}\choose{12}}$
(c)
$(3x)^{12}(-7y)^{12}{{24}\choose{12}}$
(d)
$21^{12}{{24}\choose{12}}$