Which of the following is an example of a closed system? A estuary where fresh and ocean water mix
A chemical reaction within deep ocean organisms allowing them to glow
A living single-celled organism such as a phytoplankton
A river that flows into the ocean

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When rates are expressed as a quantity of 1, such as 2 feet per second or 5 miles per hour, they are called unit rates. if you have a multiple-unit rate such as 120 students for every 3 buses, and want to find the single-unit rate, write a ratio equal to the multiple-unit rate with 1 as the second term.