Consider the following relationships among members of a family of six persons A, B, C, D, E and F:

1. The number of males equals that of females.
2. A and F are sons of F.
3. D is the mother of two, one boy and one girl.
4. B is the son of A.
5. There is only one married couple in the family at present.

Which one of the following inferences can be drawn from the above?