Certain protists have a contractile vacuole, which collects and expels excess pure water (H₂O).This is most useful for A) slime molds locomoting on dry land. B) protists in salt water maintaining a hypotonic internal environment. C) dinoflagellates evading predators. D) protists in fresh water maintaining a hypertonic internal environment. E) radiolarians moving by jet propulsion.

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Protists in fresh water need to maintain a hypertonic internal environment, which means that they have a higher concentration of solutes than their surrounding environment. This results in water moving into their cells, which can cause them to burst if they do not have a mechanism to expel excess water. The contractile vacuole allows these protists to collect and expel excess pure water, thereby maintaining a proper internal environment. Slime molds, dinoflagellates, and radiolarians do not specifically require a contractile vacuole to survive in their respective environments. Protists in saltwater require a mechanism to pump ions out of their cells to maintain a hypertonic internal environment, but the contractile vacuole is not involved in this process.

Certain Protists Have a Contractile Vacuole, Which Collects and Expels