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

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

Example Input: What occurs when substances move from areas of lower to higher concentration or when very large molecules are transported?
Example Output: active transport

Example Input: What is the sac-like organ at the end of the esophagus?
Example Output:
stomach