The threshold frequency of a metal with work function \( 6.63 \, \text{eV} \) is:
\( 1.6 \times 10^{15} \, \text{Hz} \)
\( 16 \times 10^{12} \, \text{Hz} \)
\( 1.6 \times 10^{12} \, \text{Hz} \)
\( 16 \times 10^{15} \, \text{Hz} \)