The ¹²C and ¹⁶O, Which Now Form Part of Living

The ¹²C and ¹⁶O, which now form part of living matter, were part of the cold, dark nebula from which the Sun formed. How did they get there?

A) They were part of the original Big Bang and thus part of everything formed since then.
B) They were formed by earlier generations of stars while they were on the main sequence.
C) They were formed by earlier generations of stars while they were in their red giant phase.
D) They were formed in an earlier supernova explosion and dissipated through space.