Example 3. The population of a city compared to the previous year is: in 81 it increased by 1%, in 82 it increased by 1% again, in 83 it decreased by 1%, and in 84 it decreased by 1% again. By the beginning of 85, compared to 80, it is
(A) increased by 1%;
(B) decreased by 1%;
(C) neither increased nor decreased;
(D) decreased by 0.019999%;
(E) increased by 0.019999%.
Let's assume the population in 1980 is 1, then at the beginning of 1985 it is
$$
\begin{array}{ll}
(1+1 \%) (1+1 \%) & (1-1 \%) (1-1 \%) \\
=\left[1-(1 \%)^{2}\right]^{2}<1,
\end{array} \text { so (A), (C), }
$$
(B) should be eliminated. And $\left[1-(1 \%)^{2}\right]^{2} \neq 1-1 \%$, so (B) should be eliminated. Therefore, (D) is correct.