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1 . What is the coefficient of $x^{9}y^{16}$ in the expansion of $(7x + 21y)^{25}$?

(a)

none of the above

(b)

${25 \choose 16} 7^{16}21^{9}$

(c)

${16 \choose 25} 7^{9}21^{16}$

(d)

${25 \choose 16} 7^{25}3^{16}$