Which One of the Following Examples Most Clearly Illustrates the Effect

Which one of the following examples most clearly illustrates the effect of mental set on problem solving?

A) Andy is trying to solve a physics problem on a class exam.He is so anxious that he cannot remember how to do the problem.
B) Bradley cannot find the right size of test tube to complete a chemistry lab.
C) Charlie is doing a series of addition problems.He overlooks the subtraction sign on the last problem, so he adds when he should subtract.
D) Doug is angry that another boy stole his bicycle, and so he steals a bicycle from someone else.