Given a scientific question, generate a correct answer to it.

Ex Input:
What "kind" of water may take longer to become contaminated than surface water, while the natural cleaning process may take longer?

Ex Output:
groundwater

Ex Input:
What model postulates that electrons orbit the nucleus at fixed energy levels?

Ex Output:
bohr

Ex Input:
What term in physics refers to the use of force to move an object?

Ex Output:
work