A Sample of an Ancient Sedimentary Rock Contains Slightly Less

A sample of an ancient sedimentary rock contains slightly less of the isotope carbon-13 than normal. One possible explanation for this is that

A) it was a meteorite that fell from the sky
B) some kind of radioactive decay occurred in it in the past
C) it used to be at the bottom of an ancient ocean
D) some kind of biological activity occurred in it in the past