Parts of a Supernova Remnant Become Visible

Parts of a supernova remnant become visible

A) because of radioactive elements that were created in the supernova and carried along with the remnant.
B) when large hot stars form within the gas and dust of the remnant and emit radiation, which excites the remaining gas.
C) when they collide with other clouds of gas and dust.
D) when they interact with the galaxy's strong magnetic field.