An Astronomy Student, for Her PhD, Really Needs to Estimate

Multiple Choice

An astronomy student, for her PhD, really needs to estimate the age of a cluster of stars. Which of the following would be part of the process she would follow?

A) measure the Doppler shift of a number of the stars in the cluster
B) search for x-rays coming from the center of the cluster
C) plot an H-R diagram for the stars in the cluster
D) count the number of M type stars in the cluster
E) search for planets like Jupiter around the stars in the center of the cluster