How Does Gravitational Lensing Tell Us About the Mass of a Galaxy

How does gravitational lensing tell us about the mass of a galaxy cluster?

A) Newton's universal law of gravitation predicts how mass can distort light, so we can apply Newton's law to determine the mass of the cluster.
B) The lensing allows us to determine the orbital speeds of galaxies in the cluster, so that we can determine the mass of the cluster from the orbital velocity law.
C) The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster.
D) Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.