`31.`

` V_{t t} = \frac{M}{D} = \frac{55,85}{7,78} \approx 7,1787 \text{ cm}^3`

$V_{nt} = V_{tt} \cdot 75\% = 7,1787 \cdot 0,75 \approx 5,384 \text{ cm}^3$

`V_1 = \frac{V_{nt}}{N_A} = \frac{5,384}{6,022 \cdot 10^{23}} \approx 8,94 \cdot 10^{-24} \text{ cm}^3`

$r = \sqrt[3]{\dfrac{3 \cdot V_1}{4\pi}} = \sqrt[3]{\dfrac{3 \cdot 8,94 \cdot 10^{-24}}{4 \cdot 3,14159}} \approx 1,2876 \cdot 10^{-8} \text{ cm}$

Làm tròn kết quả ta được $r \approx 1,29 \cdot 10^{-8} \text{ cm}$

`->` B