The transition region with approximately Calabi-Yau metrics Theorem 1.1 is constructed in Section 4, which is the main geometric input of this paper. From the geometric point of view, for $|t|\ll 1$, the transition region is approximately determined by the geometry of the Calabi-Yau metric on $D$, hence reduces to one lower dimension, and yet it occupies most of the volume of $X_{t}$. On the other hand, interesting topologies in $X_{t}$ are located in the two Tian-Yau ends, which have relatively small volume.