In Observations of a Double Quasar Image Produced by Gravitational

Multiple Choice

In observations of a double quasar image produced by gravitational lensing, fluctuations on the intensity of one image appear to be delayed compared with equivalent variations on the other image. Explain.

A) Rotation of the lensing galaxy has delayed the beam of light on one side compared with the other because of the Doppler effect.
B) One beam has had to pass through material with a greater refractive index than for the other beam within the lensing galaxy.
C) The gravitational field of the lensing object slows down the light on one beam compared with the other.
D) The path light takes through the gravitational lens to produce one image is longer than that for the other image.