Jane was not a good student. Her head was in the clouds(处于幻想之中) most of the time. She wanted to listen in class, but other things seemed more important: her clothes, her hairstyle, films and television. Many times she tried to work hard at her lessons, but soon she became absent-minded.
One day, her math teacher gave the class an important lesson and told her students that there would be a test the next week. She helped them review all the week. Jane seemed to listen in class, but her mind went away again.
The day of the test arrived. Jane couldn’t answer many of the questions on the exam. Mike, a very hard-working student, sat next to Jane. So Jane decided to copy his. Mike was very angry when he found Jane was copying his answers. He changed all his answers so that they were not right. Before the bell rang, Mike quickly changed his answers back into right ones. After the test, Mike turned to Jane and said, “ All those answers you copied from my paper are wrong.”
的最小正周期是 （ ）