Answer the following question given this paragraph: 

An addition reaction is a reaction in which an atom or molecule is added to an unsaturated molecule, making a single product . An addition reaction can be thought of as adding a molecule across the double or triple bond of an alkene or alkyne. Addition reactions are useful ways to introduce a new functional group into an organic molecule.


Q: What is a reaction in which an atom or molecule is added to an unsaturated molecule, making a single product?

 Choices:

- complete reaction

- homeostasis

- single reaction

- an addition reaction

A:
an addition reaction
