给出以下四个说法：
①在匀速传递的产品生产流水线上，质检员每间隔20分钟抽取一件产品进行某项指标的检测 ，
这样的抽样是分层抽样；
②在刻画回归模型的拟合效果时，相关指数$R^2$的值越大，说明拟合的效果越好；
③在回归直线方程y=0.2x+12中，当解释变量每增加一个单位时，预报变量$\hat{y}$平均增加0.2个单位；
④对分类变量X与Y，若它们的随机变量$K^2$的观测值k越小，则判断“X与Y有关系”的把握程度越大．
其中正确的说法是 （    ）

下列词语中，没有错别字的一项是（   ）

    According to a recent survey,violence did exist in schools. Students ____ (show) their fear and parents and teachers also expressed their great concern about it. ____ (expert) hope the whole society will pay more attention to the mental health of adolescents.

    Nowadays, school violence is ____ hot issue. I think this is a phenomenon（现象）, ____ calls for our great concern.We should try every effort ____ (prevent) violence happening at school for more and more students would drop out of school if their personal ____ (safe) could not be guaranteed.In fact,violence can be learned.Children learn violent behavior from adults or from ____ they see on television or on the Internet.

    If I meet with school violence,I will not answer violence ____ violence, for it will result in ____ (much、fighting.I will tell my teachers or parents about it.I think they will help me deal with it well and they will protect me from the bad guys.

    All in all, every student should behave ____ (he) and keep away from violence.