6. As shown in Figure $1, A B$ is the diameter of semicircle $O$, $A C$ and $A D$ are both chords, $\angle C A D=\angle D A B$. Then the size relationship between $A C+A B$ and $2 A D$ is ( ).
(A) $A C+A B>2 A D$
(B) $A C+A B=2 A D$
(C) $A C+A B<2 A D$
(D) None of the above, cannot compare the sizes
6. (C).

As shown in Figure 4, draw $OM \perp AC$ at $M$, intersecting $AD$ at $N$, and connect $OD$. It is easy to see that $OD \parallel AC$, $OD \perp OM$. Therefore, $AM < AN$, $OD < ND$, which means $\frac{1}{2} AC < AN$, $\frac{1}{2} AB < ND$. Hence, $AC + AB < 2AD$.