防毒面具的使用要求：作业人员在（）的环境中进行作业时，使用防毒面具。
A.不明毒物
B.不明浓度
C.有毒有害
D.陌生
参考答案：C
around the famous museum, we were taken to visit the Theme Park about five kilometers out of the city.
A．Having shown
B．Being shown
C．To be shown
D．Having been shown
轧槽是什么？
ng shown