Astronomers Identify the Main Sequence on the H-R Diagram with What

Multiple Choice

Astronomers identify the main sequence on the H-R diagram with what activity in the course of a star's life?

A) forming from a reservoir of cosmic material
B) fusing hydrogen into helium in their cores
C) letting go of a huge outer layer
D) dying
E) you can't fool me; so many stars are on the main sequence that there is no special stage in a star's life that can be identified with it