Last but not least, studying alone easily makes students distract and slowly progress. If they are not persistent and have high concentration, self-study will be extremely troublesome. There is no pressure to constraint them to study hard, so they are likely to give up and neglect. The noise and some kinds of entertainment are the big nuisances which can make their studying ineffective. Moreover, they must spend plenty of time searching materials and solving difficult problems. With teacher's assistance, they can have useful information and clue to find the answers advantageously.