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Rivers and groundwater have different chemical make-ups. The greater oxygen content in river water means that metals in the water, like iron, become oxidized and bind to other water elements; thus, they get trapped in sediment or sink to the bottom of rivers and oceans. Furthermore, river water generally passes through estuaries where sediment and pollutants are filtered out by plants, animals and plankton. Groundwater, on the other hand, contains greater concentrations of dissolved trace elements because there is not enough oxygen to oxidize them, and since it is not filtered before entering the ocean, its chemical components enter along with it.

bind = join                                          plankton = micrscopic organisms

estuaries = streams                             components = parts

The main focus of this paragraph is the difference between ____________

a. rivers and oceans         c. river water and groundwater          b. sediment and pollutants          d. iron and trace elements