(2013·厦门第一次质量检查)You had better book rooms at the hotel in advance ________ you should find no room on your arrival.

A．so that

B．even if

C．in case

D．now that

据调查甲醛是一种常见污染物。甲醛（CH₂O）中各原子物质的量之比为        ，CH₂O摩尔质量为         ， 2 molCH₂O分子中约含有            个氢原子。