One advantage of owing your own car is ________ you can go anywhere at any time you like.

A．when

B．why

C．what

D．that

某同学做物质在氧气中燃烧的实验时，要先在瓶底留些水或铺一层细沙的是    [ ]

A．红磷         

B．木炭           

C．铁丝           

D．硫磺