Let $\phi:X\rightarrow \mathbb{P}^1$ be a fibered Calabi-Yau threefold with a general fiber $F$. The following are known

- $\phi=\Phi_{mF}$ for some $m\in \mathbb{N}$, where $\Phi_D$ stands for the map associated to the complete linear system |D|.
- The fiber $F$ is either abelian or K3 surface.

How can one prove the above facts?