6. A frustum of height 8 contains a sphere $O_{1}$ with a radius of 2, the center $O_{1}$ is on the axis of the frustum, and the sphere $O_{1}$ is tangent to the top base and the side of the frustum. Inside the frustum, another sphere $O_{2}$ with a radius of 3 can be placed such that the sphere $O_{2}$ is tangent to the sphere $O_{1}$, the bottom base, and the side of the frustum at only one point. Apart from the sphere $O_{2}$, the maximum number of additional spheres with a radius of 3 that can be placed inside the frustum is
A. 1
B. 2
C. 3
D. 4
Draw the axial section of the frustum as shown in the left figure, $O_{1} O_{2}=5, M F=3 \Rightarrow O_{1} M=3 \Rightarrow O_{2} M=4$. Thus, $O_{2}$ lies on the circle with $M$ as the center and 4 as the radius. As shown in the right figure, a circle with a radius of 4 can accommodate at most 3 circles with a radius of 3 without any intersection. Therefore, the answer is $B$.