this sum_,we will have enough money for the tripA is added B added C adding C is adding 答案B为什莫

答案B是正确的.
属于英语语法的独立主格 是非谓语动词的一种用法
非谓语动词：即所谓的不做谓语的动词形式------包括 分词（过去分词ed表被动/现在分词ing表主动）/ 不定式/进行时
this sum 是独立成分,这笔钱（this sum） 是被加进来的（added）,所以是被动, 用added
这句话相当于使用非谓语动词将以下句子的简化成简单句：
并列句 This sum is added and we will have enough money for the trip.
或条件句 If this sum is added, we will have enough money for the trip.
转化的方法是 去掉连接词 （and或 if), 从句主语（this sum） 保留【因为与主句主语不一致】, is 去掉【简单句中不能有两个的谓语动词】, added 保留, 主句不变.
其实非谓语共有两种情况
1 主句与从句主语不一致, 专门的名字就是独立主格 就是上面的例子
This sum added, we will have enough money for the trip.
this sum 是独立的主语,不能省略,谓语动词is 去掉, added 与this sum 保持一致 表被动
或 If our boss gives this sum, we will have enough money for the trip.
改写为：
Our boss giving this sum, we will have enough money for the trip.
Our boss是独立的主语,不能省略, giving 与our boss保持一致 表主动
2 主句与从句主语一致
If we get the salary this month, we will have enough money for the trip !
改写为：
Getingthe salary this month, we will have enough money for the trip !
we是get的形式主语, 表主动, 与主句主语一致省略
或
If we are given the salary this month, we will have enough money for the trip !
改写为：
Given the salary this month, we will have enough money for the trip !
we是gived 的形式主语,表被动 ,与主句主语一致省略