以下哪项不是河北省海水的主要污染物？（）
A.无机氮
B.汞
C.化学需氧量
D.活性磷酸盐
参考答案：B
集艺术家、教育家于一身的戏曲大师，被誉为京剧鼻祖的是___________。
On his way home yesterday , Tom was hit on the head_____he had time to realize what was happening .
A．since
B．before
C．when
D．until
