6. If $|x|+|y|=1$, then the range of values for $S=y-2 x$ is ( ).
(A) $-4 \leqslant S \leqslant 4$
(B) $-3 \leqslant S \leqslant 3$
(C) $-2 \leqslant S \leqslant 2$
(D) $-1 \leqslant S \leqslant 1$
6.C.

As shown in Figure $5, |x|+|y|=1$
the graph is a square $ABCD, S$ reaches its minimum value at point $A(1,0)$, and reaches its maximum value at point $C(-1,0)$.