11.15 The sum of all the coefficients in the expansion of $(x-2 y)^{18}$ is
(A) 0 .
(B) 1 .
(C) 19 .
(D) -1 .
(E) -19 .
(15th American High School Mathematics Examination, 1964)
[Solution] Since its expansion is an algebraic expression in $x, y$, when $x=y=1$, it is the sum of the coefficients in the expansion. Therefore, the sum of all the coefficients is
$$
(1-2 \cdot 1)^{18}=(-1)^{18}=1 .
$$

Thus, the answer is $(B)$.