you need to change your icon; i smacked my screen no fewer than three times while typing this post, in futile attempts to kill the electronic vermin crawling on it.

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a = correct

* proper parallelism:
---- americans born before 1905 and those born since 1955
---- verbs in past perfect - parallel and context-appropriate
---- parallel constructions overall: PEOPLE had VERBed BY THIS AGE

* tenses make sense: the study deals with things that had occurred before the study was conducted, and whose relevance continued to the time of the study (also in the past). therefore, past perfect is clearly the best choice.

* meaning makes sense (deals with the proportions of certain demographics who have suffered from certain problems; compare with choice e)


choice b

* present tense 'suffer' is definitely inappropriate in the first part of the sentence: pretty much all americans born before 1905 are dead, and, even if they aren't, they passed the age of 75 a long, long time ago

* can't say 'are born' (this needs to be in a past tense, if it's tensed at all; it's better just to use the past participle, as in choice a)


choice c

* 'of them' / 'of americans' is redundant

* present perfect 'have suffered' is inappropriate, because the trend does not continue to the present (it terminated at the very latest in 1980, when the last members of the cohort mentioned turned 75)


choice d

* unjustified use of the passive voice

* 'by' is repeated 4 times, creating confusion and obfuscating parallel structure

* inappropriate use of present tense (see choice b)

* 'the six percent' suggests that
---- only 6% of americans have been born since 1955, and that
---- all of them became depressed by age 24


choice e

* fundamental shift in meaning: 'one percent of the time' is very different from the meaning of the original

* present tense is inappropriate (see choice b)

* 'did so' is awkward unless it refers to an actual action (suffering depression is not an action)

netigen wrote:Thanks Ron, appreciate the excellent explanation.

This leaves just one doubt about answer (A). How can we justify the semicolon in A. Since the second sentence is not an independent sentence because of the use of "of those"

sure it's independent; it just happens to start out with a modifying phrase. that happens all the time. here are two examples of such sentences, the first of which is random and the second of which is like the one here:
carrying a heavy load, jimmy struggled to climb the stairs.
of the ten people who responded to the survey, eight prefer brand X.