英语Things are known ------ when they unite with the oxygen of

答案是A.something is known to do something.是英语中常见的巨型,表示“大家都知道.”.这个句型可以改成：It is known that something ...
所以,这句话可以改成：It is known that things can burn when they unite with the oxygen of the air.
这句话的意思是：众所周知,当东西与空气中的氧混合在一起时,就可以燃烧.