When heated,water will be turned into steam.是什么意思

When heated,the coal gives off heavy smoke,__it difficult for people to breathe.
When heated,the coal gives off heavy smoke,______it difficult for people to breathe.
A.makes B.to make C.making D.having made
C
请问为什么用ing,这句话如何翻译,填空的地方是什么成分
为什么不能用A。makes