Today, many types of organisms live in a marine environment. when did most of the phyla of marine animals alive today first appear?

a. proterozoic
b. cretaceous
c. cambrian
d. permian

Option C, Cambriam

Explanation:

During the Cambrian period explosion of life has taken place. This period started about 543 million years ago and most of invertebrate form of life evolved during this phase among which the trilobites were the abundant ones. More than 100 types of trilobites were found during those times.  

Some of the common aquatic species that evolved during those times and are existing till today include mollusks, worms, echinoderms , sponges, chordates, algae etc.  

Hence, option C is correct

the answer is b.) hydrogen bonds between bases.

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