请教关于"extent"

57. In astronomy the term "red shift" denotes the extent to which light from a distant galaxy has been shifted toward the red, or long-wave, end of the light spec­trum by the rapid motion of the galaxy away from the Earth.

(A)  to which light from a distant galaxy has been shifted

(B)   to which light from a distant galaxy has shifted

(C)  that light from a distant galaxy has been shifted

(D)  of light from a distant galaxy shifting

(E)of the shift of light from a distant galaxy

看到这道题里面的EXTENT, 这个词我总是弄不清楚是什么意思, 看上去似乎很复杂， 很多样,

请教高手,

有其他例子关于这个词吗?

这个词在GMAT里面出现的频繁吗?