6. As shown in Figure 2. $AB$ is the diameter of $\odot O$, $\odot O_{1}$ and $\odot O_{2}$ are both internally tangent to $\odot O$, with points of tangency $C_{1}$ and $C_{2}$, and both are tangent to $AB$. Points of tangency are $D_{1}$ and $D_{2}$. The intersection point $M$ of lines $C_{1}D_{1}$ and $C_{2}D_{2}$ has the following positional relationship with $\odot O$ ( ).
(A) $M$ is outside $\odot O$
(B) $M$ is on $\odot O$
(C) $M$ is inside $\odot O$
(D) Cannot be determined
6.(B).

As shown in Figure 7, it is clear that $O, O_{1}, C_{1}$ are collinear. Let $C_{1} D_{1}$ intersect $\odot O$ at $M_{1}$, and connect $O_{1} D_{1}, O M_{1}$. It is easy to prove that $O M_{1} / / O_{1} D_{1} \Rightarrow O M_{1} \perp A B$. $M_{1}$ is the midpoint of $\overparen{A B}$. Similarly, $C_{2} D_{2}$ also passes through $M_{1}$, meaning that the intersection point $M$ of $C_{1} D_{1}$ and $C_{2} D_{2}$ lies on $\odot O$ ($M_{1}$ is $M$).
$1 \leqslant$