The system breaks down the food we eat, while the system transports the tiny food particles to each cell in the body. a. muscular; circulatory b. digestive; respiratory c. circulatory; digestive d. digestive; circulatory

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.