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.

I believe is C. A and B use the passive tense, which most fo the time should be avoided. In D, a scale for rating is not correct. In E, the likelihood is slightly more wordy that "how likely". So I go with C

I would go for D.

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.


Responding to the publics fascination....

should be followed by astronomers have developed...

"Rating the likelyhood" and "rates the likelyhood " both dont seem as right as "to rate how likely it is..".

Henc C

would go with C