\textbf{177.} The radioactive decay diagram of a sample of protium is shown below as a function of time. What is the half-life of this material in days?
\textit{[Figure: Radioactive decay graph showing number of nuclei $N$ vs. time (days). The y-axis shows $N_\circ$ and $\dfrac{31}{32}N_\circ$; the x-axis shows time in days with a marked point at 125.]}
\begin{itemize}
\item[(1)] 5
\item[(2)] 25
\item[(3)] 50
\item[(4)] 62.5
\end{itemize}
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