Answer: C
Phytoplankton are photosynthesizing microscopic organisms that inhabit the upper sunlit layer of almost all oceans and bodies of fresh water. They are agents for "primary production," the creation of organic compounds from carbon dioxide dissolved in the water, a process that sustains the aquatic food web. Phytoplankton obtain energy through the process of photosynthesis and must therefore live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, lake, or other body of water. Phytoplankton account for half of all photosynthetic activity on Earth. Their cumulative energy fixation in carbon compounds (primary production) is the basis for the vast majority of oceanic and also many freshwater food webs (chemosynthesis is a notable exception). The effects of anthropogenic warming on the global population of ph ytoplankton is an area of active research.
Explanation: Phytoplankton are microscopic organisms that live in watery environments, bo th salty and fresh. Some phytoplankton are bacteria, some are protists, and most are single-celled plants. When conditions are right, phytoplankton p o pulations can grow explosively, a phenomenon known as a bloom. Phytoplankton can also be the harbingers of death or disease. Certain species of phytoplankton produce powerful biotoxins. These toxic blooms can kill marine life and people who eat contaminated seafood.