Which choice best describes the function of the underlined sentence in the overall structure of the text?

a. It explains how decreasing plankton communities can impact the ocean's health.
b. It presents the research scientists uncovered regarding the ocean's changing color.
c. It argues that ocean color change will cause many problems, including a decrease in plankton.
d. It concludes that changes in plankton are caused by the ocean's color and climate change.