1 .
What is the coefficient of $x^{20}y^{35}$ in the expansion of $(5x - 3y)^{55}$?
(a)
$- {55 \choose 20} \cdot 5^{35} \cdot 3^{20}$
(b)
$- {55 \choose 35} \cdot 5^{20} \cdot 3^{35}$
(c)
${55 \choose 35} \cdot 5^{20} \cdot 3^{35}$
(d)
${55 \choose 20} \cdot 5^{35} \cdot 3^{20}$