Donald was not very good at math. 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 am quite good at other subjects. I will cheat in the math exam, then I won't be in trouble." "I will sit next to the boy who is best at math," he thought, "and copy down his answers." The day of the exam came, and Donald sat next to Brain Smith, who always was at the top of the class in math. 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 have decided to give a prize to the student who got the highest grade. It's difficult for me to decide who to give the prize to, however, because two students, Donald and Brian, got the same grade." "Let them share it," one of the other students said. "I thought about that," the teacher said, "but I decided to give the prize to Brian." Donald was angry when he heard this. He stood up and said. "That's not fair. I got the same grade as Brian." 'That's true." The teacher said." However, Brian's answer to Question 18 was "I don't know," yours was "Neither do I". Donald decided to cheat in the math exam because   _  .
he was always at the top of the class in math he understood the questions he was not very good at math the math teacher did not explain the questions carefully.