Please choose the correct answer to the question: A fever is a response to a disease-causing agent infecting the body. Raising one's body temperature changes the shape of enzymes in the agent so it cannot reproduce. Which statement best describes a fever?

Answer1: A fever is an internal response to an external stimulus. Answer2: A fever is an internal response to an internal stimulus. Answer3: A fever is an external response to an external stimulus. Answer4: A fever is an external response to an internal stimulus.

Answer format: answer1/answer2/answer3/answer4