I think the answer is C I think that bc I added 3 and the 2 which made it H5 and than multipled the 3 and the 2 which made it C6 than Cl
The answer is c, because we use it as an "index" to tell when the animal lived, so if it's unique to that time level in the ground we know it lived that long ago.
Answer:
D. protons that is the answer