28 -19 If $\frac{2+3+4}{3}=\frac{1990+1991+1992}{N}$, then $N$ equals
(A) 3.
(B) 6.
(C) 1990.
(D) 1991.
(E) 1992.
(77th American Junior High School Mathematics Examination, 1991)
[Solution] The left side of the equation $=3$, and the right side $=\frac{3 \cdot 1991}{N}=3$ when $N=1991$. Therefore, the answer is $(D)$.