Electrons are emitted from a conductor when the conductor is
a. subjected to a small voltage.
b. bombarded by high-speed electrons.
c. cooled rapidly.
d. exposed to weak light.

B. Bombarded by high speed electrons. 
There are already electrons with potential energy in the conductor. When it is bombarded by these high speed electrons, the potential energy is changed into kinetic energy and then the electrons are emitted from the conductor.
Hope this helps. 

Less force and energy was applied

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.