The answer to ur question is a
Commensalism is when one organism benefits off the other one without harming it. Examples are tree frogs that use plants for protection or jackals that have been expelled from packs trailing tigers and feeding on the remains of its kill. Barnacles on whales get shelter by attaching to a whales body
Possession of flagellum is not a taxonomic feature. many different unrelated organisms have flagellum. euglena, zooflagellates but also some fungi or even human sperm cells.
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A combination of sand, salt and water is an example of a heterogeneous mixture. A heterogeneous mixture is a type of combination where the constituents of the mixture are not uniform, and there are two or more distinctive phases of separation.