Cosets and Lagrange's Theorem · Medium problem

A group $G$ of order $72$ has subgroups $A$ of order $8$ and $B$ of order $9$. Since $A\cap B$ is a subgroup of both $A$ and $B$, Lagrange bounds $|A\cap B|$ by a common divisor of $8$ and $9$. What must $|A\cap B|$ be?