\textbf{178-} The decay diagram of a proton-rich substance is shown below. What is the half-life of this substance in days?
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\textit{[Figure: Graph of number of proton-rich nuclei (vertical axis: number of nuclei of proton-rich substance, with $N_0$ and $\frac{N_0}{16}$ marked) versus time in days (horizontal axis, with 22 marked). The curve is a decreasing exponential.]}
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(1) $12$\\
(2) $8$\\
(3) $6$\\
(4) $4$
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