Things are known ()when they unite with the oxygen of the air.
A.正确答案Cto have burnt
B.to have been burnt
C.to burn
D.burning
Things are known ()when they unite with the oxygen of the air.
Things are known ()when they unite with the oxygen of the air.
A.正确答案Cto have burnt
B.to have been burnt
C.to burn
D.burning