1.96 If $N=\sqrt{1985 \cdot 1985 \cdots \cdots \cdot 1985-7^{n+1}}$, then $N$ must be
(A) an irrational number.
(B) a positive integer whose last digit is 1.
(C) a number whose first digit is 5.
(D) none of the above.
(China Beijing High School Mathematics Competition, 1985)
[Solution] Let $n=1$, then $N=\sqrt{1985-7^{2}}=\sqrt{1936}=44$. $\therefore$ (A) is not true, and $(B),(C)$ are also not true. Therefore, the answer is $(D)$.