The American Heart Association has documented that the fatty and highly processed foods of the basic American diet, along with the upsurge in smoking among women, were key factors in the increasing proportion of women who died from heart disease beginning in the 1950’s, and warn that recent increases in smoking among teenage girls show that the fashion for healthier living has not entirely reversed the deleterious trends of a half-century ago.
A) were key factors in the increasing proportion of women who died from heart disease beginning in the 1950’s, and warn that
Incorrect. [singular subject] [singular verb], [modifier], and [plural verb]
B) that are the key factors in the increasing proportion of women who died from heart disease beginning in the 1950’s, and warn that
Incorrect. that X and Y that (missing verb)
C) were key factors in the increasing proportion of women who died from heart disease beginning in the 1950’s, and warns that
Correct.
D) that is the key factor in the increasing proportion of women who died from heart disease beginning in the 1950’s, and warns that
Incorrect. Same as B.
E) was a key factor in the increasing proportion of women who died from heart disease beginning in the 1950’s, and warns that
Incorrect. that X and Y [singular verb].