With respect to PQ and PQR trees, select the incorrect alternative:

     a) All PQ tree is a PQR tree, but the opposite is not valid.
     b) The children of a node R can arbitrarily permutate with each other, as well as the children of a node Q.
     c) Intersection, disjoint union and noncontained complement are operations used to calculate the completion of a collection of sets.
     d) There is always a PQR tree that represents a collection of sets, regardless of what is this set.
     e) None of the above.

Original idea by: Celmar Guimarães da Silva
Translate help: Leandro José de Bortoli