If the Root Mean Square Velocity of a Particle in a Gas

Multiple Choice

If the root mean square velocity of a particle in a gas is 450 m/s,and the particle collides with other particles every 2.0 $\times$ 10^-9 seconds on the average,then the mean free path is

A) 6.0 $\times$ 10^-7 m.
B) 7.5 $\times$ 10^-7 m.
C) 9.0 $\times$ 10^-7 m.
D) 85 $\times$ 10^-7 m.
E) 6.9 $\times$ 10^-7 m.