A water droplet with a mass of 46.8 ng (1 ng = 1 nanogram = 1E-9 g) is stationary in the air under the combined influence of gravity and a downward-pointing 56 kN/C electric field (1 kN = 1 kilonewton = 1E3 N). How many excess electrons must the water droplet have in order for the electric field to suspend it against gravity? ("Excess" electrons are those beyond the number needed for the droplet to be charge neutral.)