Most aquatic organisms are adapted to a narrow range of salinity. why must aquatic organisms living in coastal areas be adapted to survive in both low and high salinity? during high tide, freshwater mixes easily with ocean water, lowering salinity. during low tide, the ocean water becomes concentrated, increasing salinity. ocean currents bring in deep-ocean water with high salinity concentrations. temperature fluctuations can cause changes in the salinity levels of the water.

Option B, During low tide, the ocean water becomes concentrated, increasing salinity.

Explanation:

The varying moisture content and evaporation rate during tides causes variations in salinity of sea/ocean water. The surface pore water salinity reduces during high tides because of inundation and increases during low tides.  While in aquatic ecosystem, there is no as such variation of salinity and that so frequently.  

Hence, a marine species must be capable of surviving in condition of varying salinity while an aquatic species live in narrow salinity range.  

Thus, option B is correct.

a fossil fuel is a fuel formed by natural processes, such as anaerobic decomposition of buried dead organisms, containing energy originating in ancient photosynthesis.[1] the age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years.[2] fossil fuels contain high percentages of carbon and include petroleum, coal, and natural gas.[3] other commonly used derivatives include kerosene and propane. fossil fuels range from volatile materials with low carbon to hydrogen ratios like methane, to liquids like petroleum, to nonvolatile materials composed of almost pure carbon, like anthracite coal. methane can be found in hydrocarbon fields either alone, associated with oil, or in the form of methane clathrates.

the theory that fossil fuels formed from the fossilized remains of dead plants by exposure to heat and pressure in the earth's crust over millions of years was first introduced by andreas libavius "in his 1597 alchemia [alchymia]" and later by mikhail lomonosov "as early as 1757 and certainly by 1763".[4] the first use of the term "fossil fuel" was by the german chemist caspar neumann, in english translation in 1759.[5]

in 2017 the world's primary energy sources consisted of petroleum (34%), coal (28%), natural gas (23%), amounting to an 85% share for fossil fuels in primary energy consumption in the world.[6] non-fossil sources in 2006 included nuclear (8.5%), hydroelectric (6.3%), and others (geothermal, solar, tidal, wind, wood, waste) amounting to 0.9%.[7] world energy consumption was growing at about 2.3% per year. in 2015 about 18% of worldwide consumption was from renewable sources.[8]