制订职业接触限值属于哪一级别的预防工作
A．第一级
B．第二级
C．第三级
D．初始级
E．基础级
参考答案：A
—The two people speak different languages and can not communicate with each other well.
—Oh，I see. That’s ________ misunderstanding occurs.
A．when
B．what
C．which
D．why
致病力（virulence）
