I would say, C. Because an controlled variable is an experimental element which is constant and unchanged throughout the course of the investigation.
The answer to the question is A
Answer: A. All animals are multicellular, all are heterotrophic, and all lack cell walls.
Explanation:
A good index fossil is one with four characteristics: it is distinctive, widespread, abundant, and limited in geologic time. Because most fossil-bearing rocks formed in the ocean, the major index fossils are marine organisms
A. Surronding the cell membrane
