Which of the following is a common characteristic of young smokers（）

A．should be introduced, for it really works at the school where he once studied
B．should not be introduced, for it may cause disturbance
C．should be introduced though it may not work effectively
D．needn't be introduced as long as teachers don't smoke in front of children

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