某校研究性学习小组的同学为了探究呼出的气体与吸入的空气中氧气、二氧化碳和水蒸气的含量是否相同，设计了简单的实验方案，其主要操作步骤如下所示，请完成实验报告。

A check line is（）.

A.a safety line attached to a man working over the side

B.used to measure water depth

C.used to slow the headway of a barge

D.used to measure the overhead height of a bridge