单项选择题

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Supernovae are massive exploding giant stars. A supernova is known as one of the most energetic explosive events. When the explosion occurs, the resulting illumination can be as bright as an entire galaxy. It occurs at the end of a star’s lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. This will cause a blast wave that ejects the star’s envelope into interstellar space. The result of the collapse may be, in some cases, a rapidly rotating neutron star that can be observed many years later as a radio pulsar. As a result of gravitational forces acting against the nuclear structure of the core of a fuel depleted star, tremendous shock waves are generated, which causes the outside layers of the star to be blown away from the core.
Gravitational forces condensing hydrogen gas raises the temperature at the center of the star to the point where nuclear fusion is initiated. Hydrogen is fused into helium and energy is given off in the process. As more helium accumulates at the center, the temperature rises due to compression until another nuclear fusion is initiated. This time helium is converted into carbon and oxygen and additional energy is given off during the nuclear fusion.
A similar process continues with carbon and oxygen fusing to neon, magnesium and oxygen. These elements then undergo another fusion process as the temperature and pressure increase to produce silicon and sulfur. The latter two elements then fuse into iron.
During each nuclear fusion, energy is given off. However, nuclear fusion stops at iron because energy is no longer produced by fusion. The iron core collapses very quickly ( within hours or less). Since the iron core can collapse only so far and can no longer undergo fusion, it becomes extremely hot and now begins to expand rapidly. The expanding iron and the collapsing outer gases collide with each other producing tremendous shock waves which blow the outer layers away from the cure, thus causing the supernova’s gigantic explosion.
Which of the following causes the outside layer of the star to be blown away from the core