Donald was not very good at maths. He could not understand the teacher's explanations. Even when the teacher explained something a second time, Donald still could not understand it. "Never mind," Donald told himself. "I'm quite good at other subjects. I'll cheat in the maths exam, then I won't be in trouble." "I will sit next to the boy who's best at maths," he thought, "and copy down his answers." The day of the exam came, and Donald sat next to Brian Smith, who always was at the top of the class in maths. Donald carefully copied Brian's answers onto his own exam paper. At the end of the exam, the teacher collected the papers and graded them. Then she said, "Well, boys and girls. I've decided to give a prize to the student who got the highest grade. However, it's difficult for me to decide who to give the prize to, because two students, Doanld and Brian, got the same grade." "Let them share it," one of the other students said. "I have thought about that," the teacher said, "but I have decided to give the prize to Brian." Donald was angry when he heard this. He stood up and said. "That is not fair. I got the same grade as Brian." "That is true," the teacher said. "However, Brian's answer to Question 18 was 'I don't know,' while yours was 'Neither do I.'" Donald decided to cheat in the maths exam because  _  .
he was always at the top of the class in maths he understood the questions he was not very good at maths the maths teacher did not explain the questions carefully