Once an egg cell is fertilized by sperm, the cell then, as the embryo develops, it receives nourishment and eliminates wastes by transferring substances from its blood to its mother's bloo

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.