实验室测得乙烯和氧气混合气体密度是氢气的14.5倍, 可知其中乙烯的质量分数为

A．25%

B．27.6%

C．72.4%

D．75%

Do you know ________ I could pass the exam?  [ ]

A. that  

B. whether  

C. what  

D. which