问题
单项选择题
When did you check the contents of C02 cylinders（）.
A.by counting
B.by calculating
C.by weighing
D.by filling
答案
参考答案：C
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When did you check the contents of C02 cylinders（）.
A.by counting
B.by calculating
C.by weighing
D.by filling
参考答案：C