How Do We Know That at Least One Generation of Stars

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

A) The stars of the oldest globular cluster have some heavy elements.
B) They are so old that nuclear fusion in globular cluster stars has altered their chemical abundances.
C) Globular clusters have a much higher hydrogen content than expected.
D) The RR Lyrae stars found in globular clusters must be second-generation stars.
E) All globular clusters reside in the disk of the Milky Way.