Unfortunately I do not know the source of the question, hence no OA.

A study of marital relationships in which one partner's sleeping and waking cycles differ from those of the other partner reveals that such couples share fewer activities with each other and have more violent arguments than do couples in a relationship in which both partners follow the same sleeping and waking patterns. Thus, mismatched sleeping and waking cycles can seriously jeopardize a marriage.

Which of the following, if true, most seriously weakens the argument above?

(A) Married couples in which both spouses follow the same sleeping and waking patterns also occasionally have arguments that can jeopardize the couple's marriage.
(B) The sleeping and waking cycles of individuals tend to vary from season to season.
(C) The individuals who have sleeping and waking cycles that differ significantly from those of their spouses tend to argue little with colleagues at work.
(D) People in unhappy marriages have been found to express hostility by adopting a different sleeping and waking cycle from that of their spouses.
(E) According to a recent study, most people's sleeping and waking cycles can be controlled and modified easily.

It can be quite easily boiled down to A and D.
A, though, looks kind of "weak" to me, but D, from my own interpretation, strengthens the argument.

The stem:
following x+y =>
not following x+y =>

I see D as: being and not following x+y => (hostility is a bad thing, right?) how does this weaken the argument?