The Duration of the Oxygen-Fusing Stage in the Core of a Massive

The duration of the oxygen-fusing stage in the core of a massive star is considerably shorter than the duration of the carbon-fusing stage. One main reason for this is that

A) carbon is a solid, while oxygen is a gas.
B) oxygen fusion does not begin until the contracting core has reached a higher temperature than the temperature at which carbon fusion is possible.
C) the electron degeneracy inhibits the carbon reaction, but it has been removed by the time oxygen fusion begins.
D) there are more fusion shells surrounding the oxygen fusion core than there are around the carbon fusion core. The shells prevent the escape of oxygen before it can fuse.