GRE Reading Comprehension: ETS-GRE阅读ETS - V7ZGM_6621I57THW7

The passage implies that, according to the new theory of spiral-galaxy structure, a spiral galaxy can be created by supernovas when the supernovas are A. producing an underlying density wave B. affected by a density wave of spiral form C. distributed in a spiral pattern D. located in the central plane of a galaxy E. located in a differentially rotating galaxy