$33 \cdot 29$ The notation $[x]$ represents the greatest integer not exceeding $x$ (for example, $[\sqrt{5}]=2$). Let $n$ be a natural number, and
$I=(n+1)^{2}+n-\left[\sqrt{(n+1)^{2}+n+1}\right]^{2}$, then
(A) $I>0$.
(B) $I<0$.
(C) $I=0$.
(D) For different values of $n$, any of the above three cases may occur.
(China Junior High School Mathematics League, 1985)
[Solution] Since $n$ is a positive integer, the following equation holds:
$\left[\sqrt{(n+1)^{2}+(n+1)}\right]^{2}=[\sqrt{(n+1)(n+2)}]^{2}=(n+1)^{2}$, which only requires noting that $(n+1)^{2} > 0$.
$$

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