GMAT Sentence Correction (SC) | Re: The first detailed study of magpie attacks in Australia

The first detailed study of magpie attacks in Australia indicatesthat by the time they had reached adulthood, 98 percent of men and 75 percent of women born in country have been attacked by the birds.

a) by the time they had reached adulthood, 98 percent of men and 75 percent of women born in country have been attacked by the birds.
b) by the time they reach adulthood, 98 percent of men and 75 percent of women, who were born in the country, had been attacked by birds.
c) by the time they