be known to do

【问】 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，英语里没有know to do sth.的用法。

【答】 英语里虽然没有know to do sth.的格式，但却有be known to do sth.的格式：

1. He is known to be honest. (PEG)

大家都知道他很诚实。

2. 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.)