Which Statement Regarding Millikan's Oil Drop Experiment Is True

Which statement regarding Millikan's oil drop experiment is true?

A) X-rays removed electrons from fine oil droplets to produce cations.
B) The rate at which charged oil droplets fell in an adjustable magnetic field was related to the charge on an electron.
C) The rate at which charged oil droplets fell in an adjustable electric field was related to the charge on an electron.
D) The rate at which ionized N₂ and O₂ molecules fell between electrically charged metal plates was related to the charge on an electron.
E) Electrons were found to have either a positive or negative charge depending on how they were generated.