An alpha particle enters a hollow tube of 4m length with an initial speed of `1km//s`. It is accelerated in the tube and comes out of it with a speed of `9 km//s`. The time for which it remains inside the tube is
A. `8 xx 10^(-3)s`
B. `80 xx 10^(-3)s`
C. `800 xx 10^(-3)s`
D. `8 xx 10^(-4)s`
