controls are defined as
a. variables scientists observe for changes
b. things that change during an experiment
c. variables that are changed by the scientist
d. things that are kept constant during an experiment

D. Things that are kept constant.

Controls are used in an experiment where some variable is being changed. By only changing one thing and keeping the others constant they can make sure that any change in the results is caused by the variable and not the constant, since the constant didn’t change.

D

Explanation:

the rock layers with no fossils wouldn't be useful in finding out how the structure changed over time.