The "Zero-Age Main Sequence" Describes the Sequence of Stars in the Hertzsprung-Russell

The "zero-age main sequence" describes the sequence of stars in the Hertzsprung-Russell diagram that

A) contain no processed elements (elements heavier than hydrogen or helium) .
B) are just beginning to convert helium into carbon in their cores.
C) have just collapsed from the interstellar medium.
D) have just become stable objects.