In Observations of a Double-Quasar Image Produced by Gravitational Lensing,fluctuations

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 to equivalent variations on the other image.Why is this?

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