21. In 1909 Robert Millikan performed his now famous oil drop experiment. The exper- iment was set up as follows. Millikan sprayed oil into fine droplets using a atomizer. The droplets were allowed to fall under the influence of gravity through a small hole into the lower portion of the apparatus were they were viewed with a light microscope. During their fall, the drops acquired electrons produced by bombarding the air in the chamber with ionizing radiation. The electrons imparted negative charge to the drops. Millikan varied the negative charge on the lower plate to slow or even reverse the fall of the negatively charged drops. (Remember: like charges repel!) Millikan measuring the strength of the electric field required to halt the free fall of the drops, and by figuring out the masses of the drops themselves (determined by their radii and density), Millikan calculated the charge on each drop. Then, building upon the results from J. J. Thomsons cathode ray experiments which demonstrated the existence of negatively charged particles (which came to be called electrons), Millikan was able to deduce the charge a single electron. Which of the following best describe the reasoning Millikan used to determine the charge of an individual electron?

(a) Larger drops experience a greater gravitational pull and therefore require more negative charges on the lower plate to slow the fall of the oil drop. (b) Since each drop must contain an integral (or whole) number of electrons, the charge on each drop must be a whole-number multiple of the electrons charge. (c) Millikan observed a correlation between the size of the drop and the magnitude of charge required to reverse the fall of the drop. The ratio of size/charge then directly gives the charge of the electron. (d) A and B (e) A and C