用火焰原子吸收法测定水中总铬时，水样应采取怎样的保存措施？
参考答案：
加入硝酸，使pH＜2
Only _____ as an interpreter _____ how important it was to practise speaking English. [ ]
A. when did I work…I realized
B. when did I work…did I realize
C. when I work…did I realize
D. when I worked…did I realize
实施调查计划、落实调查方案是市场调查的( )。
A．开端
B．最重要环节
C．次要环节
D．得出结果的阶段
