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

Ex Input:
If no unbalanced force acts on an object the object remains at constant velocity or what?

Ex Output:
at rest

Ex Input:
What is the smallest particle of an element that still has the properties of that element is called?

Ex Output:
atom

Ex Input:
What effect in the atmosphere ensures that the earth maintains the correct temperature to support life?

Ex Output:
greenhouse effect