An ionization chamber with parallel conducting plates as anode and cathode has `5 xx 10^(7)` electrons and the same number of singly-charged positive ions per `cm^(2)`. The electrons are moving at `0.4 m//s`. The current density from anode to cathodes `4 mu A//m^(2)`. The velocity of positive ions moving towards cathode is
A. `0.4m//s`
B. `1.6m//s`
C. zero `0.1m//s`
D. `0.1m//s`
