Question: 2014 Winter Midterm - 7
What is the coefficient of $x^{9}y^{16}$ in the expansion of $(7x + 21y)^{25}$?
(a)
${16 \choose 25} 7^{9}21^{16}$
(b)
${25 \choose 16} 7^{25}3^{16}$
(c)
${25 \choose 16} 7^{16}21^{9}$
(d)
none of the above