A train car is to a train, as a is to a polymer as a molecule is to a .

The correct answer is - Polymer (macromolecules), monomer (building blocks

Explanation:

Polymers are comparable to the train as polymers are made up of several monomers linked together to carry more but can be removed from the polymer if required similarly the train cars that are comparable to the monomers or building block are joined together but can be taken off.

Macromolecules are polymers that are made up of joining monomers together and makes large molecules. Macromolecules like carbohydrates are made up of monosaccharides as monomers, proteins are made up of amino acids as monomer or building block, lipids are made up of fatty acids and nucleic acid are made up of nucleotides.