Fill in the blanks in the following seyences with "yet"or"till".
1 He learned English for three years at school.___he does not know much about it.
2 Mathematics is not very popular;___one can learn a lot in the course.
3 I did not start to learn science___became a middle school student.
4 They worked hard on the problem ___at last the solved it.
5 This class is an elective;___most of the boys like it very much.
6 He read the story ___he learned it by heart.
理由。