Question

why are algae green

Answer 1

The "green algae" is a paraphyletic group because it excludes the Plantae. Like the plants, the green algaecontain two forms of chlorophyll, which they use to capture light energy to fuel the manufacture of sugars, but unlike plants they are primarily aquatic.
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Answer 2

Your answer :

They're green because they use chlorophyll, which absorbs mostly red and blue light and reflects green.

Other plants have pigments which absorb (and use) green light too, so they have different colors. Carotenes are examples of such pigments.

Hope this helps.

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