21. The area of the triangle formed by the lines $y=5$, $y=1+x$, and $y=1-x$ is ( ).
(A) 4
(B) 8
(C) 10
(D) 12
(E) 16
21. E.

Draw the figure, and it can be seen that the height of the triangle is 4 and the base is 8. Therefore, the area of the triangle is
$$
\frac{4 \times 8}{2}=16 \text {. }
$$