Nucleus $P$ has 144 neutrons and 93 protons. Nucleus $P$ is unstable and undergoes $\alpha $-decay to form nucleus $Q$. Nucleus $Q$ then undergoes ${\beta ^ - }$ decay to form nucleus $R$.
Nucleus $P$ is represented by point $P$ on the graph.
Which point on the graph represents nucleus $R$?

A
2 )
B
3 )
C
4 )
D
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