The Spectrum of Stars in an Open Cluster Is Found

The spectrum of stars in an open cluster is found to contain spectral lines from many heavy elements; the stars are known as "metal-rich." What is the source of these heavy elements?

A) fusion reactions on the very hot surfaces of these stars
B) The heavy element content of these stars is not enhanced but the hydrogen and helium content has been depleted by intense stellar winds made up preferentially of these light elements.
C) fusion reactions in the star cores, carried to the surface by convection
D) interstellar medium, which originated in explosions of stars earlier in history that had manufactured heavy elements by nuclear fusion