How is it possible for scientists to know how much carbon dioxide is assimilated by the oceans?

A) Scientists actually do not know how much carbon dioxide is assimilated by the oceans. There are only crude estimates based off of models.
B) Scientists use ice cores to estimate how much carbon dioxide is assimilated by the oceans.
C) Scientists track this by using carbon in decayed and fossilized organisms from the deep sea.
D) Scientists can estimate how much carbon dioxide is assimilated by oceans with moderate precision because radioactive carbon dioxide from nuclear bombs can be tracked.