A nucleus that is initially at rest has nucleon number $A$. It undergoes decay and emits a proton with speed $v$, producing a new nucleus with speed $u$. The proton and the new nucleus travel in opposite directions, moving apart from each other. Which equation gives $v$ in terms of $A$ and $u$?
- A$v = \left(\frac{A}{4} - 1\right)u$
- B$v = (A - 1)u$
- C$v = Au$
- D$v = (A + 1)u$