One Possible Theoretical Explanation for the Negative Correlation Between Education

One possible theoretical explanation for the negative correlation between education and rate of Alzheimer's disease in nuns is

A) education may encourage "brain work" which increases neuronal density.
B) going to college is also correlated with family social economic status which may be the real cause.
C) nuns who are educated rise in the church hierarchy and thus lead a less stressful life.
D) There is no theory proposed to explain this observation.