Week 14 - PQ-trees

Which of the bellow statements is INCORRECT?

a) Reading the leaves of a tree T from left to right yields its frontier, denoted by
FRONTIER(T).
b) A null tree (which has no nodes) is NOT considered as a PQ-tree.
c) A reversal of a P node is a equivalence transformation.
d) A reversal of a Q node is a equivalence transformation.
e) None of the above.

Translated by: Raphael Cristofaro
Original idea by: Moacy Barros Correia da Silva
(question number 071)