Light from the sun travels through space to earth's atmosphere.

which will light waves do when they move from empty space into matter made up of different gases?

a. move at the same speed
b. move faster as they transmit
c. slow down and refract
d. speed up and reflect

I think the correct answer from the choices listed above is option C. The light waves slow down and refract when they move from empty space into matter made up of different gases. Light does not travel as fast through a non vacuum, and any change in velocity results in a refraction. 

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.