Our English teacher worked late into the night,____the exam

Our English teacher worked late into the night,____the exam for the following wekks.
A.to be prepared B.prepared C.preparing D.was preparing 为什么选C?

这就是一个典型的伴随状语. 用ing形式来表示伴随.
定义:伴随状语是指状语从句的动作伴随主句发生,它的特点是：它所表达的动作或状态是伴随着句子谓语动词的动作而发生或存在的. 　　
例如： 　　
①He sat in the armchair,reading a newspaper. 　　他坐在扶手椅里读报. 　　
②All night long he lay awake,thinking of the problem. 　　他整夜躺在床上睡不着,思考着那个问题.
伴随状语几种表示方法
一、使用分词形式 　　The dog entered the room, following his master(这条狗跟着主人进了屋). 　　The master entered the room,followed by his dog(主人进了屋,后面跟着他的狗). 　　
二、用with复合结构 　　The little girls were playing with snow with their hands frozen red(小女孩们在玩雪,手都冻红了). 　　
三、用独立主格结构 　　The little boy goes to school, the little dog accompanying him every day(这小孩每天去上学,那条小狗陪伴着他). 　　
四、用形容词 　　
Crusoe went home, full of fear(克鲁索满怀恐惧地回家). 　　
Confident of the victory the players are fighting hard(运动员们对比赛夺胜满怀信心奋力拼搏). 　　
The match will be broadcast live(这场比赛将作实况转播). 　　
He left home young and came back old(他少小离家老大回). 　　
五、用名词 　　
He went away a beggar but returned a millionaire(他讨吃要饭离家,腰缠万贯回归). 　　
He who had been twice prime minister of the country, died civilian(他曾两度出任这个国家的总理,而死的时候却是一个平民). 　　
六、用介词短语 　　
The girl came back to her mother in tears.(这女孩眼泪汪汪地回到母亲身边). 　　
How can you go to the wedding party in rags(你怎能衣衫褴褛地去参加婚宴)? 　　
I went home out of breath(我上气不接下气地回家).
伴随状语的判断
　　The dog entered the room, following his master.一般在分词短语（现在分词表主动 过去表被动）前有逗号的是伴随状语.