Hydrothermal vents spew metal-rich waters into the deep ocean, but they do not significantly affect average ocean salinity over long timescales because:
a) The metal-rich waters mix quickly with seawater and are diluted.
b) The deep ocean currents carry the metal-rich waters away from the surface.
c) The metals precipitate out of the water column before affecting salinity.
d) The metal-rich waters evaporate before contributing to ocean salinity.