What did Robert Millikan's oil drop experiment prove?

AAtoms are composed of positive charges.
BIt determined the actual mass of the electron.
CMatter can be conserved.
DAtoms can be neither created nor destroyed.

Verified Answer :

Robert Millikan's oil drop experiment proved that it determined the actual mass of the electron. Robert Millikan was a well known experimental physicist.

Answer:

Robert Millikan was a well known experimental physicist. Robert Millikan's oil drop experiment proved that it determined the actual mass of the electron.