增加：在缺词处加一个漏字符号（∧），并在其下面写出该加的词。

删除：把多余的词用斜线（\）划掉。

修改：在错的词下划一横线，并在该词下面写出修改后的词。

    If you are on the street, you will see all kinds of people passing by. It's not rare to see people with bad manner. Some throw away rubbish casually. Some spits in public. Some even sit or lie on the ground, completely ignore others' feelings. People feel really badly about all these behaviors.

    To be honesty, smoking in public places makes me really annoying. As it is known to us all, smoking does harm for people's health. Not only is it bad for the smoker himself, but also it is threat to the public health. It was hoped that an effective way can be found to educate these people to behave well in public.

下列事件是必然事件的是（　　）