The study, called the National Lung Screening Trial

aspirant2011 wrote:The study, called the National Lung Screening Trial, focused on a specific high”risk group: 53,000 current and former heavy smokers, aging from 55 to 74, which had
smoked for at least 30 pack”years

C.group: 53,000 current and former heavy smokers, aged 55 to 74, who smoked for at least 30 pack”years

D.group: 53,000 current and former heavy smokers, aged 55 to 74, who had smoked for at least 30 pack”years

E. group: 53,000 current and former heavy smokers, who were aged 55 to 74,and who had smoked for higher than at least 30 pack”years

[spoiler]I have a doubt in option C i.e I went for this option because of the use of word "current" in "current and former heavy smokers" as I felt that the smokers are current ones + former and therefore, we don't require past participle had but the correct answer given is D ..........can someone clear my this doubt???? [/spoiler]

vineeshp wrote:Answer D looks fine.

who did the study target? Some ppl who had done some thing. Doesnt this sound better?

David@VeritasPrep wrote:Got a PM on this one...

The study "focused" which is the past tense as mentioned above. Now you speak of "current smokers" but the study focused on current smokers when that study was done. We are not told the year but it is clearly past tense, so let's say what if the study was done in the year 2000? The term current smokers is talking of current when the study is done, so current smokers in the year 2000 in our example.

So you see that "current smokers" is not a problem. And the study having "focused" in the past tense requires the past perfect for "had smoked."