关于情态动词一般不改变原则的特例讨论。请大家指教！

第比利斯

Responding to the public’s fascination with-and sometimes
undue alarm over-possible threats from asteroids, a scale
developed by astronomers rates the likelihood that a particular
asteroid or comet may collide with Earth.
A.    a scale developed by astronomers rates the likelihood
that a particular asteroid or comet may
B.    a scale that astronomers have developed rates how
likely it is for a particular asteroid or comet to
C.    astronomers have developed a scale to rate how likely
a particular asteroid or comet will be to
D.    astronomers have developed a scale for rating the
likelihood that a particular asteroid or comet will
E.    astronomers have developed a scale that rates the
likelihood of a particular asteroid or comet that may.
正确答案，C
这道题我在C和E中徘徊。 虽然C感觉很好。但是因为GMAT的SC有一个原则是情态动词一般不改变，不然就改变原句的语气了。 所以E很难排除。
我的疑问，是否也有特例。
谢谢大家。

第比利斯

不好意思，答案是C呢。。。。。

sj33333

how likely是很口语的说法，一般不出现在书面语中。