In a Certain Gas the Molecules Are 5

In a certain gas the molecules are 5.0 *10^-9 m apart on average, have a mean free path of 5.0 *10^-6 m, and have an average speed of 500 m/s.The rate at which a molecule has collision with other molecules is about:

A) 10 ^{- 11} s ^{- 1}
B) 10 ^{- 8} s ^{- 1}
C) 1 s ^{- 1}
D) 10⁸ s ^{- 1}
E) 10¹¹ s ^{- 1}