In Attempting to Solve the "Nine Dot Problem" Few Individuals

Multiple Choice

In attempting to solve the "nine dot problem" few individuals attempt to draw a line outside the imaginary boundary surrounding the nine dots.This is not a rule associated with the problem but is imposed by the problem solver.What is this an example of

A)  trial-and-error learning 
B)  functional fixedness 
C)  mental set 
D)  unnecessary constraints