Answer the following question given this paragraph: 

In this equation, two substances combine to make a single substance. This is a composition reaction. Two different substances react to make two new substances. This does not fit the definition of either a composition reaction or a decomposition reaction, so it is neither. In fact, you may recognize this as a double-replacement reaction. A single substance reacts to make multiple substances. This is a decomposition reaction.


Q: What type of reaction causes two substances to combine to make a single substance?

 Choices:

- component reaction

- spontaneous mutation

- metabolism

- composition reaction

A:
composition reaction
