从事使用高毒物品作业的用人单位应当至少对高毒作业场所进行一次职业中毒危害因素检测；至少（）进行一次职业中毒危害控制效果评价。
参考答案：每半年
传染性非典型肺炎病人早期不能用抗病毒药。
A man aboard a vessel，signaling by raising and lowering his outstretched arms to each side，is indicating（）.
A.danger，stay away
B.all is clear，it is safe to pass
C.all is clear，it is safe to approach
D.a distress signal
