6. As shown in Figure 1, in $\triangle A B C$, $A B=A C, C M$ 

bisects $\angle A C B$, intersecting $A B$ 

at point $M, A D$ 

$\perp B C$ at point $D$, 

$M E \perp B C$ at point 

$E, M F \perp M C$ intersects 

$B C$ at point $F$. 

Then the value of $C F-4 D E$ is ( ).

(A) positive number
(B) 0
(C) negative number
(D) cannot be determined.
6. B.

As shown in Figure 6, extend $C A$ and $F M$ to intersect at point $P$. Then $C F = C P$.

Draw $M N \parallel B C$ intersecting $A D$ and $A C$ at points $H$ and $N$ respectively. Then
$$
\begin{array}{l}
M N = \frac{1}{2} C F, D E = M H . \\
\text { Since } A B = A C, A D \perp B C \\
\Rightarrow D E = M H = H N = \frac{1}{4} C F .
\end{array}
$$

Therefore, $C F - 4 D E = 0$.