Answer the following question given this paragraph: 

A theory is constantly tested. If some data does not agree with the theory, the theory can be revised. If the theory cannot match the data, it is thrown out. That being said, sometimes a theory is well-established; it has a lot of evidence to back it up. A well-established theory is unlikely to be overthrown. It may just need to be modified a bit. But science does not prove anything beyond a shadow of a doubt.


Q: In science, what is constantly tested, sometimes revised, sometimes thrown out and sometimes well-established?

 Choices:

- a technique

- a study

- a variable

- a theory

A:
a theory
