Which of the following sets of elements all have exactly one valence electron (simplified main-group rule)?

• A. $F$, $\mathrm{Cl}$, $\mathrm{Br}$, $I$
• B. $H$, $\mathrm{Li}$, $\mathrm{Na}$, $K$
• C. $\mathrm{Be}$, $\mathrm{Mg}$, $\mathrm{Ca}$, $\mathrm{Sr}$
• D. $C$, $\mathrm{Si}$, $\mathrm{Ge}$, $\mathrm{Sn}$

Answer: (B)