If macro molecules are known as polymers why is each subunit known as a monomer?

In the text it says “Most macromolecules are made from single subunits, or building blocks, called monomers. The monomers combine with each other using covalent bonds to form larger molecules known as polymers. In doing so, monomers release water molecules as byproducts. ... In the process, a water molecule is formed.” I hope this helped

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