$30 \cdot 27$ An integer that is the square of another integer is called a perfect square. If $x$ is a perfect square, then the next perfect square is
(A) $x+1$.
(B) $x^{2}+1$.
(C) $x^{2}+2 x+1$.
(D) $x^{2}+x$.
(E) $x+2 \sqrt{x}+1$.
(30th American High School Mathematics Examination, 1979)
[Solution] Let $x=n^{2}(n \geqslant 0)$ be a perfect square, then the next perfect square is
$$
(n+1)^{2}=n^{2}+2 n+1=x+2 \sqrt{x}+1 .
$$

Therefore, the answer is $(E)$.