$$
\begin{array}{l}
A=\left\{y \mid y=-x^{2}+1, x \in \mathbf{R}\right\}, \\
B=\{y \mid y=-x+1, x \in \mathbf{R}\},
\end{array}
$$

If the sets are defined as above, then $A \cap B$ equals ( ).
(A) $(0,1)$ or $(1,1)$
(B) $\{(0,1),(1,1)\}$
(C) $\{0,1\}$
(D) $(-\infty, 1]$
$$
-1 . \mathrm{D} \text {. }
$$

From $A=(-\infty, 1], B=(-\infty,+\infty)$, we get $A \cap B=(-\infty, 1]$.