A charged bead is capable of sliding freely through a string held vertically in tension. An electric field is applied parallel to the string, so that the bead stays at rest at middle of the string. If the electric field is switched off momentarily switched on:

Question

(a) the bead moves downwards and stops as soon as the field is switched on

(b) the bead moves downwards when the field is off and moves upwards when the field is switched on

(c) the dead moves downwards with constant acceleration till it reaches the bottom of the string

(d) the dead moves downwards with constant velocity till it reaches the bottom of the string