Which one of the following selections, if true, points out the most critical ... ...

Which one of the following selections, if true, points out the most critical weakness in the method used by the author to investigate birth order characteristics?
(A) Last born children are typically more laid-back and calm than their older siblings.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Exercise (Inductive Reasoning)