In Figuring Out the Evolutionary Tracks on the H-R Diagram

In figuring out the evolutionary tracks on the H-R diagram, astronomers

A) look at fossils from many times in Earth's history in a museum
B) observe one star over the course of several decades
C) make model stars on a computer and then follow how their characteristics will change with time
D) use big telescopes to observe how the outer layers of stars fall in during the protostar stage
E) send robot probes to the Sun