All algae are a. heterotrophs b. autotrophs c. parasitic d. anaerobes

All algae are Autotrophs.

Option B

Explanation:  

Most of the algae plants are known to be from the autotrophs, or the better term for them is known to be photoautotrophs. Autotrophs are the organisms which is able to produce their own nutrition that is why algae are able to make their own food substances. The term photo is used because of the light with the help of the light algae is able to make their own nutritional benefits and substances. Only higher plants like algae are able to make their own nutritional benefits.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."