1.98 If $P=\sqrt{\underbrace{111 \cdots 1}_{2 n \uparrow}-\underbrace{222 \cdots 2}_{n \uparrow}}(n$ is a natural number). Then
(A) $P$ is an irrational number.
(B) $P=\underbrace{111 \cdots 1}_{n \uparrow}$.
(C) $P=\underbrace{222 \cdots 2}_{n \uparrow}$.
(D) $P=\underbrace{333 \cdots 3}_{n \uparrow}$.
(E) $P=\underbrace{777 \cdots 7}_{n \uparrow}$.
(Chongqing City, Sichuan Province, Junior High School Mathematics Competition, 1983)
［Solution］Notice that
$$
\begin{array}{c}
\underbrace{111 \cdots 1}_{2 n \uparrow}-\underbrace{222 \cdots 2}_{n \uparrow}=\frac{10^{2 n}-1}{9}-\frac{2\left(10^{n}-1\right)}{9} \\
=\frac{10^{2 n}-2 \cdot 10^{n}+1}{9}=\frac{\left(10^{n}-1\right)^{2}}{9}, \\
P=\frac{10^{n}-1}{3}=\underbrace{333 \cdots 3}_{n \uparrow} .
\end{array}
$$
Therefore, the answer is $(D)$.