— Li Lin is very bright and studies hard as well.

— It's no _____ he always gets the first place in any examination. [ ]

A. question

B. doubt

C. problem

D. wonder

有两瓶没有标签的无色气体，只知它们分别是氧气和二氧化碳．请设计一个实验方案将它们区分开来，并将实验操作、预期现象和结论填写在下表中：