下列物质中属于纯净物的是( )
A.煤
B.石油
C.水煤气
D.甲烷
答案:D
长枕埋入式无碴轨道施工技术的关键为()。
The killer was caught at a pub ______ he was surfing the net, ______ relaxed us all.
A.which, that
B.where, which
C.where, that
D.that, which