This is from the GMATCLUB grammar book (Related thread: /forum/gmat-grammar-book-relative-clauses-98556.html):
Had questions on #6 and #7 from the exercise
6. Dragons breathe columns of fire. Dragons fly through the night sky.
My Answer: Dragons, which fly through the night, breathe columns of fire. NR
is my answer also correct? It does not match the orig ans. the OA says this is Restrictive. I dont quite understand this because if we remove the phrase "which fly through the night" it does not change the meaning of the original statement. I think the original statements are not specific to any kind of dragons and are applicable to all dragons. Can someone pls explain?
7. Free Willy was a cute move. Free Willy was a story about freedom.
the OA for this is: Free willy, that was a story about freedom, was a cute movie. NR
the answer sentence has a "that" in it. Can this still be a NR? does this not violate the rule - "that can ONLY be used in restrictive clauses."?
I've never looked at the exercise to which you refer, but it seems to make little or no sense. I agree with all of the points you've made.
For (7), the official answer is definitely grammatically incorrect. On the GMAT there will never be a comma between the subject of the sentence and the word "that". As you note, only "which" follows a comma in that fashion.
I'm not sure what to say about (6); if all you're given is two facts about the universe and no other information it's almost impossible to say what's restrictive or non-restrictive. It would be equally correct to say:
Dragons, which breathe columns of fire, fly through the night sky,
since there's no context for the information.
Now, if some dragons breathe fire and some don't, we might say:
The dragons that breathe columns of fire fly through the night sky
and if some dragons fly through the night sky and some don't, we might say:
The dragons that fly through the night sky breathe columns of fire.
However, with only the two sentences you've noted, there's no reason to believe that the information is meant to be restrictive (if anything, we can infer that it's non-restrictive, since it seems to apply to all dragons).