Consider two boxes containing ideal gases $A$ and $B$ such that their temperatures, pressures, and number densities are the same. The molecular size of $A$ is half of that of $B$ and the mass of molecule $A$ is four times that of $B$. If the collision frequency in gas $B$ is $32 \times 10^{18} /s$, then the collision frequency in gas $A$ is . . . . . . $/s$.

• A
  $32 \times 10^{18}$
• B
  $4 \times 10^{18}$
• C
  $2 \times 10^{18}$
• D
  $8 \times 10^{18}$