Minick Rosing's analysis of microscopic globules of carbon found in rocks 3.7 billion years old showed that they were somewhat higher in ¹²C as opposed to ¹³C.This is significant because ________.

A) ¹³C radiometrically decays into ¹²C, which lets us calculate the age of these rocks
B) life could not have arisen in an environment with excessive ¹³C
C) these carbon globules contain visible remains of cell walls and nuclei
D) living things that fix carbon preferentially incorporate ¹²C over ¹³C
E) meteorites must have brought ¹³C to the early Earth