Much of the Mass in Galaxies and Clusters of Galaxies

Multiple Choice

Much of the mass in galaxies and clusters of galaxies is dark matter. Which of these techniques is NOT useful for detecting evidence of this dark matter?

A) measuring the speed of stars in their orbits around a galactic center
B) gravitational lensing of very distant galaxies by a nearby galaxy (or cluster) and its accompanying dark matter
C) observing the aftermath of a collision such as that in galaxy cluster 1E0657-56
D) gravitational waves