It has been shown that children who smoke have certain characteristics. Compared with non-

smokers they are more rebellious, their work deteriorates as they move up school, they are more likely to leave school early, and are more often delinquent and sexually precious. Many of these features can be summarized as anticipation of adulthood.

There are a number of factors, which determine the onset of smoking, and these are largely psychological and social. They include availability of cigarettes, curiosity, rebelliousness, appearing thought, anticipation of adulthood, social confidence, the example of parents and teachers, and smoking by friends and older brothers and sisters.

It should be much easier to prevent children from starting to smoke than to persuade adults to give up the habit once established, but in fact this has proved very difficult. The example set by people in authority, especially parents, health care workers, and teachers, is of prime importance. School roles should forbid smoking by children on the premises. This role has been introduced at Summerhill School where I spent my schooldays.

There is, however, a risk of children smoking just to rebel against the rules, and even in those schools which have tried to enforce no smoking by corporal punishment there is as much smoking as in other schools. Nevertheless, banning smoking is probably on balance beneficial. Teachers too should not smoke on school premises, at least not in front of children.

In this passage the author puts an emphasis on ______.

A．the effect of smoking among children

B．the difficulty in preventing children from smoking

C．the reasons why children start smoking among children

D．the measures to ban smoking among children