In our class, when the bell rang and the teacher closed his book, it was a ________ for everyone to stand up.[ ]

A．signal  

B．chance

C．mark  

D．measure

下表中列出了一定压强和温度下每立方厘米活性炭所能吸附的常见气体的体积，分析表中数据与气体的组成、性质的关系，你认为下列结论正确的是

C．气体是化合物时，易被吸附       D．气体的相对分子质量越大，越易被吸附