The Distinct Blue Color of the Nebulosity Around Stars in Young

Multiple Choice

The distinct blue color of the nebulosity around stars in young clusters,such as the Pleiades,is caused by

A) light emitted by very hot gas which has been heated by collisions in the interstellar gas.
B) preferential scattering of blue starlight by small dust grains in the interstellar material.
C) atoms of gas emitting light by fluorescence,having been excited by ultraviolet radiation from hot stars.
D) halos caused by refraction of starlight in ice crystals in the nebula,similar to halos seen occasionally in Earth's atmosphere.