光学甲烷检测仪零点漂移的原因是什么？
参考答案：
（1）仪器空气室内空气不新鲜；
（2）对零地点与测定地点温度和气压不相同；
（3）瓦斯气路不畅通。
Every possible means ____ to save the wounded soldier up to now.
[ ]
A. was tried
B. were tried
C. have been tried
D. has been tried
在环境科学上，pH小于（）的雨水称为酸雨。
