Sentence Correction (SC) | Re: Q. The four-million-year-old fossilized skeleton known as

There's an idiom in there (not correctly completed in the original sentence): "Lucy is SO X... THAT <Y is true or Y is the case as a result>. There's no "THAT Y" in the original sentence. Eliminate A. Note the basic meaning of the nonunderlined part: Lucy is SO SMALL compared with many other skeletons... (THAT). Whatever follows "that" will have to provide the reason why we're emphasizing how small Lucy is.

Examine the other choices:
B) SO small (compared to skeletons
...

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