在自由基聚合反应中，易于分解出初级自由基的物质称为（）。
A、单体
B、引发剂
C、活性中心
D、活性自由基
参考答案：B
Teachers and nurses who deal with children are obliged to report cases of suspected child abuse to authorities.
A. reminded B. expected C. compelled D. requested
