A scientist stated the following hypothesis: If I change the temperature of water in a fish tank, it will affect fish breathing rate.
Identify the independent variable:
fish breathing rate
type of fish
type of fish tank
temperature of water

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.