Answer: (E) It cannot be known whether it was nuclear deterrence that worked, or some other factor, such as a recognition of the economic value of remaining at peace.
Explanation:
The argument that a third world war between superpowers has not happened because nuclear weapons act as a deterrent infers that it could only be because of nuclear weapons that a war has not broken out yet there could be a number of other reasons this could be the case.
This therefore presents a flaw in the argument that needs to be further investigated and until we can definitively state that there weren't other factors involved, we cannot say with certainty that the peace between the Superpowers is only down to nuclear deterrence.