Please choose the correct answer to the question: Kendall studied the ways in which human body systems work together. He compared the respiratory and circulatory systems. In which way are these two systems similar to each other?

Answer1: They both bring oxygen to the body. Answer2: They both send messages to the body. Answer3: They both digest nutrients for the body. Answer4: They both pump blood through the body.

Answer format: answer1/answer2/answer3/answer4
Answer: answer1