What does H20 stand for

One hydrogen atom, two oxygen atoms

Actually - It's two hydrogen atoms and one oxygen atom.

The number after the element symbol is the number of atoms of that element. Hence H2 means 2 hydrogens, and the O has no number so it is just one of that atom.

H2O is also known as water ;)

The main role of dna in the  cell is the long-term storage of information. it is often compared to a blueprint, since it contains the instructions to construct other components of the  cell, such as  proteins  and rna molecules.the main function of rna is to carry information of amino acid sequence from the genes to where  proteins  are assembled on  ribosomes  in the  cytoplasm. mark brainliest, my work is good.

The littoral zone. . .