TOEFL Listening: ETS-TOEFL听力机经 - U4OKN15LGVH533D9P$

What does the professor say about the origin of the crab nebula? A. It was formed when interstellar gas clouds combined. B. It was created by an exploding star. C. It did not emit when it was first formed. D. It was formed in the same way as many reflection nebulae.