$1 \cdot 36$ If $x^{2}+x-1=0$, then $2 x^{3}+3 x^{2}-x$ equals
(A) 0.
(B) 1.
(C) -1.
(D) Cannot be determined.
(2nd "Five Sheep Cup" Junior High School Mathematics Competition, 1990)
[Solution] Given $2 x^{3}+3 x^{2}-x=(2 x+1)\left(x^{2}+x-1\right)+1$,
then according to the problem, we have
$$
2 x^{3}+3 x^{2}-x=1 \text {. }
$$

Therefore, the correct choice is $(B)$.