One Proposed Test for Determining Life on Mars Is to Measure

One proposed test for determining life on Mars is to measure carbon isotopes in rock samples. How could this indicate life?

A) Biological organisms preferentially concentrate ¹²C and not ¹³C.
B) Biological organisms preferentially concentrate ¹³C and not ¹²C.
C) Radiogenic¹⁴C dating can be used to determine when an organism ceased to take in carbon from the atmosphere.
D) A carbon compound, benzene (C₆H₆) , is produced by biological organisms on Earth. A detection of benzene on Mars should indicate life.