核心问题
Things are known ____ when they unite with the oxygen of the air. A. burning B. to burn C. burn D. being burned
答案为 B(to burn)，但我认为答案是错误的。
词典解答
应当牢记 be known to do sth. 是一个很常用的句型,例如:
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He is known to be honest. (PEG) 大家都知道他很诚实。
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He was known to have taken part in the Long March. 人们都知道他参加过长征。
(参见 know sb. to do sth.)