1 .
What is the coefficient of $x^{15}y^{5}$ in the expansion of $(-3x + 5y)^{20}$?
(a)
$- {20 \choose 5} \cdot 3^{15} \cdot 5^5$
(b)
${20 \choose 5} \cdot 3^{15} \cdot 5^5$
(c)
${20 \choose 5} \cdot 5^{15} \cdot 3^5$
(d)
$- {20 \choose 5} \cdot 5^{15} \cdot 3^5$