Which of the Following Is Not a Way That Astronomers

Which of the following is not a way that astronomers can find how much dark matter there is in cluster of galaxies?

A) measuring the speeds with which the galaxies are moving around, to estimate the mass of the cluster needed to keep them from moving away
B) measuring how much slower galaxies in a rich cluster or supercluster are moving than the Hubble Law would predict
C) observing x-rays from the hot gas in the cluster, and using the amount of x-ray radiation to estimate the mass of the cluster
D) using gravitational lensing effects to make a map of how much mass a cluster of galaxies contains
E) observe the radio waves coming from all dark matter; from the strength of the radio waves from each cluster, estimate the amount of dark matter needed to produce them