An ionisation chamber with parallel conducting plates as anode and cathode has `5 xx 10^7` electrons and the same electrons are moving towards anode with velocity `0.4 m//s`. The current density from anode to cathode is `4 muA//m^2`. The velocity of positive ions moving towards cathode is

Options

A. `0.4 ms^-1`
B. `1.6 ms^-1`
C. zero
D. `0.1 ms^-1`.