Estuaries serve which important role for marine ecosystems?
A. providing spawning and nursery grounds for many marine animals
B. adding oxygen to the ocean water
C. removing carbon dioxide from the ocean water
D. removing salt from the ocean water

Ivan is looking at a cross section of a stem taken from a vascular plant. he sees two different types of vascular tissue: the xylem, which is closer to the center of the stem, and the phloem, located around the xylem, closer to the outside of the stem. how do these two structures work together in a living plant?