Sentence Correction (SC) | Re: The study, called the National Lung Screening Trial, focused on a spec

This one is relatively easy.
Since "heavy smokers" are people correct reference in modifiers is "who". Hence A, B are out leaving with C,D, and E.
Looking at E "who were aged" is incorrect since it means ages in the past. and the last phrase is unnecessarily wordy => "higher than atleast 30"?
Leaving out C and D. Since "had been smoking" is correct to denote the first action D is correct.

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