11.16 Expand $\left(x^{2}-2 x y+y^{2}\right)^{7}$, the sum of its coefficients is
(A) 0 .
(B) 7 .
(C) 14 .
(D) 128 .
(E) $128^{2}$.
(16th American High School Mathematics Examination, 1965)
[Solution] From $\left(x^{2}-2 x y+y^{2}\right)^{7}=\left[(x-y)^{2}\right]^{7}=(x-y)^{14}$, in its expanded form, let $x=y=1$ which gives the sum of all coefficients.
And $(1-1)^{14}=0$.
Therefore, the answer is $(A)$.