Why Do We Know That at Least One Generation of Stars

Why do we know that at least one generation of stars formed and died before the Milky Way's globular clusters formed?

A) All globular cluster stars have some amount of heavy elements.
B) They are so old that nuclear fusion in globular cluster stars has altered their chemical abundances.
C) Globular clusters are 9 to 10 billion years old.
D) No globular cluster is older than 12 billion years.
E) All globular clusters reside in the disk of the Milky Way.