The circle shown has center $O$, and diameters $AB$ and $CD$. The length of arc $AC$ is three times the length of arc $CB$. What is the central angle $∠BOD$ in radian?$\frac{1}{4}𝜋$$\frac{3}{4}𝜋$$\frac{1}{2}𝜋$$𝜋$
