An Astronomer Using a Refracting Telescope Sees an Image of Two

Multiple Choice

An astronomer using a refracting telescope sees an image of two gas clouds with identical shapes but displaced slightly from each other.One is composed of blue light and the other red light.The most likely explanation is:

A) the Doppler effect.
B) chromatic aberration.
C) dust absorption.
D) interstellar scattering.